If a tank pulls too much and there is no keeping the alive, just be like, "Sorry, but that was too much, can we pull a bit less?" (I like to use 'we' over 'you' because saying 'can you pull less?' could make it sound like your trying to start a fight if they read it wrong, which can happen because text doesn't have any voice tone to it and can easy be misinterpreted) If they continue to pull the same with the same results, just leave, because honestly, there isn't much you can do if you've already been nice about it or you feel like your talking to a wall.

I rarely have problems in DF, if too much is pulled and there is a wipe, the tank will usually pull less on the next try or ask if it was too much. Otherwise, if your not comfortable with speed running, I would suggest a marco that you can click the moment you enter, asking for small - medium pulls (or whatever you comfortable with) with a little <se.#> so that way it alerts them of it, and it is instant.