A switched-capacitor (commutating) filter can put a potentially very
sharp notch at a frequency determined by its clock. I assume in this
AFC that you'll have access to the frequency you're trying to filter
out. If you use an 8-capacitor version of the filter, you just need to
clock it at 8* the freq you want to remove.

You CAN also design a continuous-time band-stop filter.

(Soap-box comment: seems like this would all be a lot easier if you
just digitized the signal at 8k samples/second, making detection and
removal of fs/8 pretty easy... or just transmit the SSB with a
suppressed carrier that you don't have to worry about removing.)

Cheers,
Tom