Then that's their problem. You have explained the situation politely; if they don't like it, they know what to do about it. (They just don't want to.) I had to learn really early on how to deal with "Don't interpret that!" -- teachers love to say stupid crap and then get worried about it being interpreted -- and it can be touchy. With the right person, humor is an option; I've used "Too late, I'm just that fast!" or, for a little more message, "You say it, I sign it!" But with the wrong person, you have to take them aside afterwards, gently explain it all, and still be prepared to be resented for doing your job.
I spent a lot of extra time before and after work, when I interpreted at colleges, answering teachers' questions about interpreting OR having to instruct them myself. (I remember a teacher who wanted us to speak everything we signed so that he could tell if we were getting the message across. My team interpreter and I had to have a conference after the class to explain about ASL interpreting.)
Bottom line, if you have explained the rules, now it's just a matter of diplomacy in dealing with the fact that they don't like it. Not one of the best parts of the job for sure, but remember you are doing the right thing! You are being ethical and appropriate. They are being doodyheads.