I conveniently forget people's coffee orders if they don't tip.
why do you forget their orders just because they dont give any tip?
They shouldn't order anything in the first place if they cant give any tip.
You can't expect customers to give you tip all the time. It's wrong to not fulfill their orders that they already paid for.
I think it will teach them a lesson in dealing with me.
You get paid to do your job, not mess around. Not everyone tips before receiving their drink. You could get fired if your boss finds out what you keep doing.