Well. I don't this it's the wisest idea. But, here you go:
1. Do everything you need to do to the soon to be frozen account.
2. Log into the account you want to freeze.
3. Go to this link:
http://www.neopets.com/remacct.phtml
4. BEWARE! This is your last chance. Do you really want to get rid of your account forever? If you do, fill in the information asked and hit 'Remove My Account.'
5. Your account is now frozen forever.