Is there a way to reset everyones post count to 0?
You may use User Pruning setting in AdminCP.
@ Yaldaram - that would delete all posts, which he probably wants to do.
But, in the case where he wants to keep all posts but set all user post counts to 0, then he could run this query:
UPDATE `mybb_users` SET `postnum` = 0
I did not want to delete all posts. That would have been catastrophic. So I marked NaXuh as best answer. Though i thank you for your reply Yaldaram.