Though there are a number of issues with the built-in ATC, erroneous altitude assignments are the worst. As has been mentioned, ignoring improper assignments gets the job done most of the time, but inappropriate altitude assignments for light GA aircraft can get ridiculous (and become absolutely nauseating if early in a flight, as they will not stop for a long, long time).
Under those circumstances, If it’s available from the menu, I cancel IFR, then request “flight following”. Flight following works quite well and has never interfered with altitude assignments - it also behaves very much like IFR communications. When close enough to my destination to begin descent for the approach, I reactivate the original IFR plan and continue the approach.