1. Put your mailing list address on hold
2. Duplicate your mailing list into a new list (with a new name, obviously)
3. Secure the list - do not allow public posts to it. This is how you got into the problem you're in
4. Send out an apology email to your list, and let everyone know that the problem is under control
5. Try to track down who was spamming your list and take appropriate action. Chances are you'll never be able to track them down, but you may as well try.
If you don't have a reliable way to create a secure mailing list, use a reputable service like
Constant Contact.
Good luck.