I'm worried because my ex-crush is now crushing on me, but I don't like him because he is such a jerk!, what do you suggest?

What's the worst? The greatest danger is a possible stalking situation. Read up on this and if this is the case, respond appropriately. Otherwise, this is part of life that we all go through -- almost everyone has a failed relationship and most of us have someone unwilling to let go. Seek the advice of mutual friends, be kind but firm, and set appropriate limits.