So how do I tell someone

To go and stick their asinine problem where the sun don't shine.

You go and do a batch of voluntary work only for some idiot to start digging you out.

I can't believe this guy - I've sorted out bucket loads of problems for him/them in my own time and all he can do is wag the finger at me. I ought to break that off and shove it up his - oh well you get the picture.

Never teach a pig to sing - it takes ages and upsets the pig :-)