Yes you can delete a Sims account but unfortunately it isn't as simple as just logging in and finding the 'delete' button, for some reason EA decided not to include that, maybe they will look at that soon.
You will need to contact them direct but if you go to the help section of the EA website Monday to Friday you will be able to chat directly with EA customer care or if at the weekend you should be able to send them a message through there.
If you live in the US there is also a toll free number (also Monday - Friday) which is 866-543-5435 Press# 3 for The Sims 3
Apparently they are super helpful, but please remember that you won't be able to use the game code again to create another account, so you might want to talk to them about an alternative code.
