If some accounts were incorrectly merged, then it was probably a result of noticeable cross-voting hinting that they were, in fact, a single user trying to "game" the system. But: if a very small number of questions from this are causing you big problems then as a gesture of goodwill, let me know which and I will disassociate them from your account. To be clear - I really mean "I can count them without taking my socks off" here...
Just please stop editing them all!
Re the actual question(s) here:
Why user cannot freeze all upcoming answers and comments to his old questions?
because in general, that is part of an important feedback loop, that is intended to help you understand the question, etc.
One other way is to make the names of downvoters and upvoters public such that I can start to flag those users if necessary.
That is not going to happen. If they downvote a poorly written question, then... don't write poorly written questions! Downvotes don't actually sting very much.