OP is a server. Servers don't have a whole lot of authority. In some restaurants and restaurants chains, they have none. Kicking people out is largely up to the managers or owners. If these regulars are known to be shitty people and aren't banned, chances are the manager cares more about the money they bring in than the money they cost them by being unpleasant. Servers can get in trouble, up to and including being fired for kicking customers out, or even just trying to. So yeah, OP didn't ask them to leave because they value keeping their job over not having to provide service to the shittiest customers.

ding ding ding. Found the person who's never worked in food service. Unless the server is dualing as a manager on that shift they don't get to pick and choose who leaves or who you wait on. And there's nothing you can do to prove there was money on the table before they sat themselves, unless there's a camera pointed at that specific one, in which case the manager would probobly just make them give it back, not kick them out.

But spitting in someone's meal does nothing if they don't know about it. The food will taste the same, only YOU would get some sort of tiny satisfaction, but they would still be dicks towards you as if nothing happened, because in their reality, NOTHING HAPPENED. There's no point.

bit alas there is not much you can do without getting fired.. which you definitely would of your boss found out you spit in food. what one gets out of it is knowledge. they know they spit on it. even if no one else does.