If a person takes someone in front of him as a sutrah then that person moves away, should he move to a nearby sutrah?
Question

While I am praying behine someone else, taking him as a sutra, if he leaves what should I do? Should I move forward or stay where a was?.
Praise be to Allah.Firstly: The sutrah is mustahabb according to the majority of fuqaha’, and some of them are of the view that it is obligatory. 
It says in al-Mawsoo’ah al-Fiqhiyyah (24/177): It is Sunnah for the worshipper, if he is praying alone or is leading others in prayer, to have a sutrah in front of him that will prevent people  from walking in front of him, and enable him to focus properly on the actions of the prayer. That is because of the report narrated from Abu Sa’eed al-Khudri (may Allah be pleased with him), that the Prophet (peace and blessings of Allah be upon him) said: “When one of you prays, let him pray facing towards a sutrah and draw  close to it, and not let anyone pass in front of him.” And he (peace and blessings of Allah be upon him) said: “Let one of you use a sutrah when he prays, even if it is an arrow.” This includes when one is travelling and when one is at home, and it includes both obligatory and naafil prayers. 
The idea behind it is that the worshipper should refrain from looking beyond the sutrah, and focus properly so that his mind will not wander; the idea is also to prevent others committing sin by passing directly in front of him. 
The command in the hadeeth is to be understood as meaning that it is mustahabb, not obligatory. Ibn ‘Aabideen said: It is makrooh to omit it. There are reports to show that it is not obligatory such as the report narrated by Abu Dawood from al-Fadl ibn ‘Abbaas (may Allah be pleased with him) who said: The Messenger of Allah (peace and blessings of Allah be upon him) came to us when we were in the desert and he prayed in the desert with nothing in front of him. 
The Hanbalis said something similar. Al-Bahooti said: That is not obligatory because of the hadeeth of Ibn ‘Abbaas (may Allah be pleased with him) who said that the Prophet (peace and blessings of Allah be upon him) prayed in the open with nothing in front of him.  
It is mustahabb according to the Hanafis, and the well known view of the Maalikis, for the imam and the person praying alone if they think that people will walk in front of them. Otherwise the sutrah is not prescribed in those cases. And it was narrated that Maalik enjoined it in all cases. This was also the view of Ibn Habeeb and was favoured by al-Lakhmi. 
The Shaafa’is said that it is Sunnah in all cases, and they did not mention any limits. 
The Hanbalis said: The sutrah is Sunnah for the imam and the person who is praying alone, even if they do not fear that anyone will pass in front of them. 
For the person who is praying behind an imam, it is not mustahabb for him to use a sutrah, according to consensus, because the sutrah of the imam is the sutrah for those who are praying behind him, or because the imam is a sutrah for him. End quote. 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said, after quoting the evidence for both points of view: The evidence of those who say that the sutrah is Sunnah, who are the majority, is stronger, and this is the more correct opinion. End quote from al-Sharh al-Mumti’ (3/277). 
Secondly: 
There is nothing wrong with taking a person who is praying or sitting in front of you as a sutrah. Then if he leaves and you find another sutrah close by, such as a wall or pillar, or another worshipper, you may move towards it, and his moving will be forgiven, because it is for the sake of the prayer. But if there is nothing nearby, you should complete your prayer as you are, and stop anyone who tries to walk in front of you. 
It says in al-Mudawwanah (1/202): Maalik said: If a man is praying behind an imam and he missed part of the prayer, and the imam says the salaam, and he has pillars on his right and on his left, there is nothing wrong with moving back and standing behind the pillar on his right or on his left, if it is close by, to use it as a sutrah. He said: The same applies if it is front of him and he moves forwards towards it, so long as it is not far away. He said: The same applies if it is behind him; there is nothing wrong with moving backwards, if it is close. He said: But if the pillar is far away from him, he should pray where he is, and he should try to prevent those who want to pass in front of him as much as he can. End quote. 
And Allah knows best.