I can't count how many times I've gone to Wendy's and asked for a fish sandwich with no tartar sauce. And every time it comes WITH tartar sauce! And I'm not deaf.
Sounds like there are just a bunch of lazy workers at that fast food place. If these people can't understand plain english, there's no saying their ability to read is any better. I would do what you did, and find a different restaurant. If they don't make sure their servers are giving good customer service, they don't deserve your business...