How do I remove this type of screw/nail (Cadet Heater FWC wall can)
It is more than likely that those are large nails into the framing lumber to the side of the heater housing. 

There would be a number of ways to remove those. 


Use a Dremel tool with a cutoff wheel to cut the head off the nail. Then you can use a nail set or punch to drive the remaining body of the nail further into the side stud to allow the housing to be removed. 







Use a pry bar that has a nail puller slot. Pound that in under the nail head to get it to start coming out. It may be necessary to use a hammer and pound the metal near the nail head toward the side to free up just a little space under the nail head to let the pry bar get a bite.





A small to medium cold chisel and a sledge hammer can be used to chop the head off the nail. It is possible that the cold chisel may start to draw the nail out just enough that you could then remove it with the pry bar.






I am not promoting any particular brand of tools here. Just ones I've had good luck with working on this type of problem.