You are correct and the dps should be using Quelling Strikes before they burst in their opener for sure. I do this and just "got into the habit from the start". I think some players when challenged like this take offence and shut down on people trying to give them help most of the time. All I can offer is to ask as nicely as possible and that is about all you can do. If it comes down to it you can "teach them a lesson" but then that sets things up to be tense in the run and people RAGE quitting. When I tank I never have issues with this and I am sure that dps are not using their abilities as well. I think you could just stand to be "more" prepared with your threat so that this doesn't happen. Up to you in the end though. Good luck.