I'm going to have to tow someone from my spot
What makes you think that towing them is okay?
Because I want to park there later
While is expected to respect someone's assigned parking, its rude to tow someone else without asking them to move first. You could end up damaging their car during the tow or they may not be able to find their car.
What if I just asked them to move.
I think that you should give them some time to move then. It is not okay to take matters into your own hands like that. Just ask them and then wait for them to move.