Switching on and off, changing frequencies, playing low quailty recordings might sound obvious.

Anoraks can tell, you tell something is an in car tx from the fluttering signal dissmiss it. If you had a stationary transmitter, playing old pirate recordings it would sound more pirate like it was something, if it was playing back to back music it would sound like an in car tx that was stopped.
They could just get in a car and see, that it had greater range, if they also had another reciever else were they could tune some distance away and hear it, making it an unsafe thing. Anoraks are more of a danger they are constantly tuning about, you could get one who is like an official radio inspector, its better noone knows.

Stations that sound legitimate seem to stay the longiest
people don’t care about weather something is licenced or unlicenced they like it.
I remember years ago when i started to hear energy, usually pirates had biscuit tin transmitters not something you could hear as far away, with it having such a big tx and been heard so far away, with it sounding so professional, with there been no internet then, i though it was licenced, meant to be there, it broadcast pretty much continuously for 7 years, they were more relaxed there it was still something.
fip in brighton transmitted 10 years, used frequencies used by french station with the same rds, i don’t think anyone though it shouldn’t be there, people just liked it. I think it was more someone who paid to use the spectrum, more likely found it shouldn’t be there and that was it.

I suppose it would I think be difficult to find a station that you liked in the uk that was out there, put on air on the same frequency else were.
I suppose you could test with a blank carrier on the annoraks radio they won’t notice as carriers are quite common, it maybe takes the fun out of it. Maybe radio 2 transmitted on a different frequency, the anorak would be fooled, noone expects anything but radio 2 on those frequencies between 88-91, but safer to not transmit at all.

tef radios tune up in 100khz steps, thats were stations are usually centered if something on a half step would be invisible to the anorak, near enough to the tx, most peoples radios it would still be recieved even with their radio tuned off 50khz, invisible to the rak. If you could center it abit off frequency, the autoseek function would not work if someone was scanning up the band. I can’t think of anything else right now, interesting this dosen’t go anywhere, I think making something as legitimate as possible is good advice.