The easiest thing to do is type the following (one at a time) in the command line (Ctrl+Shift+C):
testingcheatsenabled true
moveobject on
Now go into Build/Buy mode and you should be able to click on the fireman is if he were a piece of furniture. So click him to pick him up and then hit delete.
He should then be gone. If you don't want to totally get rid of him, then pick him up and place him outside your house, like the front lawn. That should also fix him. However, if he still doesn't go away, I'd go with the deleting him - he's borked.