I let my supervisor deal with it. It´s his job to assign tasks to me and say no to customers if there is not enough capacity. The real problem with those tasks is that time needs to go there and the time is taken from the tasks I am supposed to accomplish with a deadline. If the supervisor is not involved he just sees that my tasks get delayed without proper cause.
We are required to write up our time, so even when I do it we charge for it (which means if push comes to shove, the executive has to respond for wasting resources on private issues). When my supervisor is comfy with me delaying work items for that setup, I am comfy too. Experience shows that he usually is not comfy with delay on his projects.