As others have said, neither is more correct, nor is either more preferable in an objective sense.
If you were to indicate when you blocked the account you must use the latter, but otherwise either is perfectly fine.
For me, personally, I would prefer the latter in most cases - it's less formal and sounds (to me) more like natural speech, and is (slightly) less verbose.
I, too, would drop the comma in this context, for whatever that's worth.
EDIT
I would use the former if I had blocked the comment before and had to do so again:
"I have previously blocked the user account..."
english.stackexchange.com/users/143/cori
If this was a software message, as it seems it may be, I would not use that tense.
Instead I would say: The user account has been blocked and all comments deleted.
Otherwise in a regular conversation the latter sentence (minus the comma) would be correct.
english.stackexchange.com/users/687/jjclarkson
I have blocked the user account, and deleted the comments.
This is the present perfect tense.
I blocked the user account, and deleted the comments.
This is the past tense.
Which one is preferable, or more correct?
Either one is correct.
There is a difference between the two, but without context it would be silly to try to explain it.
english.stackexchange.com/users/None/
I have blocked the user account, and deleted the comments.
I blocked the user account, and deleted the comments.
Which one is preferable, or more correct?
There are differences between North American English and British English.
For North American English, this is the present perfect of current relevance/importance.
Often, unless the speaker wishes to illustrate that a past action is important to the current situation, the simple past is used.
There is also a myth that we never use a past time adverbial with the present perfect and most often that is true,
the reason, the "job" of the present perfect is not to reflect on the past and using past time adverbs would take away from that ideal.
However, there are situations where this is not followed.
As I said they are not at all common.
english.stackexchange.com/users/2015/Dan
Both are correct, although I would consider the comma optional.
The word "have" is unnecessary.
I would personally choose the second sentence, unless context dictated otherwise.
(For example, if the text surrounding the example sentence was particularly formal.)
english.stackexchange.com/users/161/Goodbye Stack Exchange
One of the tasks I have as site maintainer of a site is deleting spam reported from users. (The spammer account is blocked too, as part of the process.) Which is the preferable sentence, or the most correct one, I should use? I have blocked the user account, and deleted the comments. I blocked the user account, and deleted the comments.
"I have blocked the user account" versus "I blocked the user account"
english.stackexchange.com
<word-choice>
english.stackexchange.com/questions/1780
english.stackexchange.com/users/252/kiamlaluno
It is generally agreed that both sentences are correct, although they are different. However, the difference will depend on the context. There may be differences in usage between British and American English. Both are correct, but they represent different tenses and usage will depend on context.
It is generally agreed that both sentences are correct, although they are different. However, the difference will depend on the context. There may be differences in usage between British and American English. Often, unless the speaker wishes to illustrate that a past action is important to the current situation, the simple past is used.   If you were to indicate  when  you blocked the account you  must  use the latter, but otherwise either is perfectly fine.  
Either one is correct. If you had to indicate  when you blocked the account, then you must use the latter, but otherwise either is perfectly fine.   Although either sentence is generally considered correct, the exact answer will depend on the context of the message. People may naturally tend towards the second option, and there may also be regional differences. If this were a software message however, neither message would be considered acceptable.
Both are correct, but the latter sounds more natural. In a message I would say: The user account has been blocked and all comments deleted.  This is the difference between the present perfect tense "I have" and the past tense "I blocked". Unless the speaker wishes to illustrate that a past action is important to the current situation, the simple past is used. 
 If you were to indicate  when  you blocked the account you  must  use the latter, but otherwise either is perfectly fine.   Often, unless the speaker wishes to illustrate that a past action is important to the current situation, the simple past is used.  
Both sentences are correct. Many people would more naturally opt for the second of these two sentences. There are differences between the two, but the correct sentence will depend on the context. There may be regional differences in usage. Neither of these sentences might be appropriate as a software message.