1. Go to http://lists.squeakfoundation.org/listinfo/squeak-dev
2. Enter the e-mail address at which you are receiving the list
3. Click "Edit Options"
4. Click "Email My Password to Me"
5. Wait for the E-mail
6. Go back to the Edit Options page
7. Enter the password
8. Click Unsubscribe
Steps 4-7 are only necessary if you don't know your password.
Note:You have to use the exact address which you are subscribed under. If you don't know, look in the headers of a posting you receive from the list to figure it out.
If all else fails, contact the list owners at the adress given at the above web page.