If this is an offline account you can just re-lock the achievements. But by doing this they will all show up as "Secret - Unlock this achievement to find out more about it!" Whatever the message is.

If you have connected to xbox live you're SOL and resetting your gamerscore yourself is impossible unless Microsoft does it for you because of cheating or whatever.

yea thats the prob. i cant pay for mems with diamond. anyone got an account with diamond that theyd let me sign into? you dont have to let me if you dont want to but if you could it'd be a big help

Pusha_T wrote Mod your Gamerscore then tell somebody on Xbox support that you modded your GS and hope your account deosn't get perm ban.

Really stupid idea but may work. lol

funny you should say that...i tried that and they said they wount reset me haha. they thought i was lying and they dont do resets on request lol.
its alright tho i made a second account