As a player who's tanked in multiple MMOs you just learn to deal with it. Sure it's slightly annoying, but you're a tank, and above being a damage sponge, it's your job to keep hate and take it back from anyone who wants to steal it from you. There's plenty of tricks that will allow you to keep aggro off other players (barring that you don't have too much of a gap between item levels). To tell the truth, the fact that people don't necessarily follow the plan allows the DF to remain interesting. Being able to keep aggro on enemies despite them being targeted separately is what will net you those sweet commendations at the end.

Realize that yes, while other people can make your life easier for you by attacking your target, you must also realize that it doesn't give you the right to suddenly kick or stop doing your job when that player doesn't necessarily follow your carefully laid out battle strategy. Especially since this isn't your raid group, or even people who've necessarily played with a tank who's marked their mobs before, they are players of varying levels of skill and gear that are willing to play with people who they don't know. Keep in mind that while it may only take a split second for you to queue, it takes dps and sometimes even heals 15-30 minutes just to queue up for a dungeon even if it's a random.