Replace Query

[Murty]

Hey guys, i posted this in the mysql forums but noone has replied yet, just wandering if you guys might be able to help me.
Thanks

 Hey guys, 
Im new here and i have one question. 
I have vbulletin forum installed on my website and obviously it uses mysql database. 
Here is my situation. 
I have this 'Thanks' system incorporated into it, which has added a field ('post_thanks_amount') to the 'post' table. When a user is thanked (in a post) the value in the 'post_thanks_amount' field goes up by one. 
A couple users abused this system and i want to reset the 'post_thanks_amount' value back to 'o' their 150 or so posts. (obviously to big to manually go through and do it). How can i do this. 
I guess the following fields would need to be used. 
- 'username' 
- 'post_thanks_amount' 
(they are both in the 'post' table')