Do they have the right to declare their father legally incompetent and deny him access to his wealth, because he has grown old and senile, and he squanders the money?
Question

                My father is eighty-four years old; sometimes he is in his right mind and sometimes he does not recognize his children. Some people from Oman started to visit him, and they bring him some drinks which he buys from them for a large amount of money, that may reach as much as one million.

Do his children have the right to declare him legally incompetent and deny him access to his wealth?
Praise be to Allah.
      The 
      majority of scholars regard it as permissible to declare an adult man 
      legally incompetent and deny him access to his wealth, if he is not of 
      sound mind and cannot take proper care of his wealth, or if he will waste 
      it, or he has lost his mind, or he is mentally ill. 

    The 
    one who does not handle wealth properly is regarded as foolish or 
    feebleminded by the fuqaha’, because he wastes money and spends it 
    inappropriately, in ways contrary to those indicated by Islamic teachings 
    and common sense. 

    Based 
    on that, wastefulness that stems from foolishness is a valid cause for 
    denying access to wealth, according to the majority of fuqaha’, the Maalikis, 
    Shaafa‘is and Hanbalis. It is also the view of our two companions, Abu 
    Yoosuf and Muhammad ibn al-Hanafiyyah, and is the basis of fatwas issued by 
    them. 

    End 
    quote from al-Mawsoo‘ah al-Fiqhiyyah, 4/194 

    It 
    says also in al-Mawsoo‘ah al-Fiqhiyyah, 17/92: There is no difference 
    of opinion among the fuqaha’ regarding declaring an insane person legally 
    incompetent and denying him access to his wealth, whether his insanity is 
    permanent or temproary, and whether it is strong or mild. 

    
    Similar to one who is insane is the man who has grown old and senile to such 
    an extent that he cannot handle his wealth or organize his affairs 
    properly. 

    Ibn 
    Qudaamah said: Ahmad said: The elderly man who has lost his mind should be 
    declared legally incompetent and be denied access to his wealth; his case is 
    like that of one who is insane, because as a result of that he is unable to 
    handle his wealth in a proper manner and protect it. Hence he is akin to the 
    minor and feebleminded.

    End 
    quote from al-Mughni, 6/610 

    Al-‘Imraani 
    said: The evidence for it being allowed to declare the feebleminded, the 
    minor and the insane legally incompetent and deny them access to their 
    wealth is the verse in which Allah, may He be exalted, says (interpretation 
    of the meaning): “But if the debtor is of poor understanding, or weak, or 
    is unable himself to dictate, then let his guardian dictate in justice” [al-Baqarah 
    2:282]. “Of poor understanding” includes the one who squanders his money 
    and the one who is denied access to his wealth because he is a minor.

    
    “Weak” includes the elderly and senile, minors, and the insane. 

    
    Allah, may He be exalted, tells us that their guardians should act on their 
    behalf with regard to their rights and duties, which indicates that it is 
    allowed to declare them legally incompetent and deny them access to their 
    wealth.

    End 
    quote from al-Bayaan fi Madhhab al-Imam ash-Shaafa‘i, 6/207 

    Ibn 
    Abi Shaybah (may Allah have mercy on him) narrated in his Musannaf 
    (6/291) that ‘Abd al-Malik ibn al-Mugheerah said: Najdah wrote to Ibn 
    ‘Abbaas to ask him about the old man who has lost his mind or become senile. 
    He wrote back to him, saying: If he has lost his mind or become senile, he 
    is to be denied access to his wealth. 

    Al-Mirdaawi 
    said: al-Maroodhi said: I think that the son should declare his father 
    legally incompetent and deny him access to his wealth if he spends 
    extravagantly, or wastes it on immoral pursuits.

    End 
    quote from al-Insaaf, 5/333 

    Based 
    on that: 

    If it 
    becomes apparent that your father has lost his mind or the way he conducts 
    his financial matters is objectionable, then there is nothing wrong with you 
    referring the matter to the Islamic judge (qaadi) to examine his case and 
    rule whether or not he is to be declared legally incompetent and denied 
    access to his wealth, according to what the qaadi finds from his examination 
    of the case and the comments of experienced specialists. 

    That 
    is because declaring a person legally incompetent and denying him access to 
    his wealth is a judicial matter that can only be done on the basis of a 
    ruling from the Islamic judge, as is the view of the majority of scholars. 

    Ibn 
    Qudaamah said: No one can declare a person legally incompetent and deny him 
    access to his wealth except the judge (qaadi). This is the view of ash-Shaafa‘i… 
    because spending extravagantly may vary and is a subjective matter, and each 
    case must be decided on its own merits. Therefore, it cannot be decided 
    except by the opinion of a qaadi. 

    End 
    quote from al-Mughni, 6/610

    See 
    also al-Mawsoo‘ah al-Fiqhiyyah al-Kuwaitiyyah, 17/96-97 

    And 
    Allah knows best.