Steve Ray has a list of 35 loaded Questions for "Bible Christians" (quotation marks his)(link to the whole list). This is number 18/35. I'm trying to provide the answers in a common format, for easy reference.
18) Since each Protestant must admit that his or her interpretation is fallible, how can any Protestant in good conscience call anything heresy or bind another Christian to a particular belief?
By appealing to the authority of Scripture.
1) Everyone (not just "Protestants") has to admit that his interpretation is fallible. You don't become infallible by joining Rome.
2) When we say that something is heresy, we are not saying that we are infallible. Roman Catholics aren't claiming that when they call something heresy, neither are we.
3) The Roman Catholics judge heresy by the standard of their church's teachings - we use Scripture. Our standard is better than theirs, but the basic principle involved is the same.