Bad A/C Issue...

Another one on the del sol the "mechanic" sold me...

The car's A/C doesn't work. The guy who sold it to me said that he checked the compressor to make sure it kicked on, and that all it needed was a charge, but I don't see how he could have been certain about the compressor after I've now driven the car for a few days.

The fan blows but I can't get the air system to shut off. The control knob is broken, but I can't get it shut off using ANY method. So if it doesn't shut off then how would he have been able to turn it back on to hear the compressor kick on?

Am I facing some serious issue with this system? I know I sound really clueless with this car (and I am) but I could really use some advice on what I'm facing here. Should I try to charge the system myself? I did this on my last car, but my last car didn't have an air conditioner that wouldn't shut down.

Any advice is greatly appreciated.