"I have blocked the user account" versus "I blocked the user account"

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..."

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.