Getting a copyright is really simple. If you have a sound recording of the works in question, simply go the the copyright page, get form SR, fill it out, include the $30 fee in the form of a check or money order, and the recording, send it in. If all you have is the lyrics, then get form PA.
Here are some tips:
The $30 fee may change as of June 2002
Send any requests via FedX, or UPS. The reason for this is that they are not accepting any requests from the USmail because of the anthrax thing.

If you really want these guys to stop using your music, copyright it. Even if the recording is of them.