The FCC is overworked and under financed to patrol these illegal FM stations and immediately shut them down.
First step they usually take is to send a letter to the address and tell them to cease operations. Then the next step is to go down the studio or where operation has been traced and they knock on the door telling them to immediately shut down and a following warning if they operate again...they will return with a sheriff and seize equipment.
The FCC's job is not only looking for illlegal FM transmissions....they have to patrol , inspect , issue and allocate other radio operations in the public safety, satellite, Shortwave, Amateur ham, etc. with exceptions like the 225-300 mhz which is military government...they are stressed. It's like seeking every CBer in the 70's that swore , squished, or hooked up a linear to their transmissions back in the 70s'-80's.
It took Radio Free Santa Cruz 8 years operating 24/7 at 100 watts, before they were taking down. This could be at least a 3 month process even if WJBR is on top of it which helps speed up the process.