It's happening to me also. On GMRS 22 (.725). On Tuesday nights we will have a net here and the unknown station will key up and start interfering with the rollcall.
On several occasions the person/persons will dead keys for the duration of the net, and I hear their carrier over the station trying to check in.
I got to a point where I actually told the station where I was and when I did the station stopped transmitting. Apparently, the person listening lived near me, I did see some kids with radios in hand was running into a house across the street. Now that I know who's throwing dead carriers I know where to look.