Shouldn't have paid for the food. Also, fuck em. Leave a shitty review on as many places as you can complaining about food poisoning and unsanitary conditions (saw someone drop food on the floor and put it back on the plate or something). Also throw in a fun phone call to the health department stating that you got incredibly sick after eating their food - even if an inspection comes up clean it doesn't look good that they're having an inspection.

I'm honestly to a point where I'm okay with embracing dirty tactics if they push this shit. They wanted to play dirty by denying you service, throw it right back in their face.