I just had the case, that the OP removed the "accepted answer" after one month because he needed my help again. http://stackoverflow.com/questions/4499476/how-can-i-clear-a-sqlite-database-each-time-i-start-my-app
I lost 15 reputation and got it again after I helped but this might be used for blackmailing. Ok 15 rep isn't worth blackmailing but my answer is the only one to the topic, there are 18 comments to help the OP and the accepted answer was there for one month.
I suggest that the accepted answer can be only changed to another answer and not "removed" which wouldn't result in such a behavior.
(hope its clear what I mean)
Update: To clarify: I don't want that a "accepted answer" can't be changed or removed at all. I just suggest to do a time frame within the accepted answer can be changed/removed completely. After this time frame it should be only possible to change the accepted answer to another answer but not removing it all. Another possibility is a "punishment" (like -1 rep for a down vote) when the accepted answer is only removed and not changed after the time frame. Hope that clears it up...