I understand your frustration. My first "bad" paramedic call was at a racetrack where there was already a private ambulance staged. They, however, didn't have a paramedic so it was "MY" call. The guy was entrapped due to no doors on racecars. Also he had no feeling below his collarbone. They had nothing on him. The racetrack is at least 15 miles from the station that I responded from....So I get there and fire crews won't let me access my patient. I just went with it for the most part. I did get in and start a line and talk to the patient though while they continued to cut. So I have 1000NS hanging on a 16g cath TKO. Some male nurse keeps opening my line up. I told him probably 5 times to cut it out. His response is that the patient is a trauma patient and trauma patients need fluid. I keep calmly telling him that if I could hear lung sounds or get a BP i may give fluid but since I dont know what shape his lungs are in I'm not giving fluid. Eventually, after the man was extricated and I had just had enough, I yelled "everyone hush!" That got there attention and from there on out I ran the call. As we left the racetrack some of the guys told me that they respected me for controlling the scene. If I had done it in the beginning, it wouldn't have gotten as out of control as it did.
I've since learned that if I walk into a call assertive I get much more respect from the bystanders. I work with a man who is an intermediate and is about 45y/o, so I have to assert myself so people know I'm running the show. The longer you let it go on, the harder it is to correct.
I think the biggest difference I made was just how I approached. Head up, confident, and talk assertively. If you act unsure of yourself they are going to be unsure of you also. If that is not enough I would tell the fire department This is my scene, things will be done my way. I have a similar fire department with similar thought process. If what they are doing could hurt my patient or myself I am sure to say something, otherwise I keep my mouth shut and do it my way. I have worked at my service long enough and we are small enough to know which fire guys are actually helpful and which ones are more of a hindrance. I give directions to the ones I trust and the other ones I send to do traffic control or something similar.
I hope this helps and god luck to you!