Former foodservice worker (in the US) for several years here. Servers do not spit in customers' food; that would be psychopathic behavior that no manager would tolerate and grounds for immediate termination, and the type of person who would actually do that is the type of antisocial person who is going to struggle in any job. If a known bad tipper ends up in a server's section, she'll roll her eyes in private, perhaps grumble to her co-workers, then go put on the customer service smile and serve them as normal.

That said, please tip. Note that (where legal) servers are generally paid below minimum wage because it's expected their income will be supplemented with tips. Consider the tip not a bribe but as a payment for a service you were provided which, at least in the current cultural climate, is not being tacked on to the actual bill; you may not like that and want that cultural norm to change, but it hasn't changed yet and is unlikely to in the near future. If tipping really offends you, then order pick-up instead of dining in, or just cook at home.