My old beater

I have a old 240 Volvo that keeps running when I turn it off. I have taken it to my mechanic about this a couple of times and he took some module out from somewhere around the glove box and cleaned it off. This fixed it for a while but it started again so he went to a junk yard and got another one of these and replaced it with a used one. It continues to do the samething, running on after the key is turned off. He told me to pull the fuse out to the ignition. I have to do this from time to time and it worked. Stopping the car from running. I would then replace the fuse and back to usual. Well my brother borrowed my car and I wrote out the instructions for him clearly instructing what fuse to pull ectera. When I got the car back he had the same problem with the car running on after the key was turned off. He pulled the fuse. Problem solved. He then forgot to replace the fuse and we have not had a problem with it running on after the car was turned off since. The fuse is on the dash. Can I just leave this fuse out? It does not appear to be affecting anything else? This is my reserve car that I use only occassionally. I am keeping it to replace the current Volvo that i am driving that just has 275,000 miles. Thanks Tammara