If they are downloaded ringers:
Go to the My Content folder and then to Ringers. Scroll down to the ringer you want to delete Click on Options and then erase.

This should be the same for the "included" ringers that came loaded on the phone.

Go to menu "get it now" then "get tunes..." and there pick up this folder what did you want and on the left site its should by Erase

I cant figure out how to erase sounds off of my samsung sch-a950