"Never try to teach a pig to sing; it wastes your time and it annoys the pig."
- Robert Heinlein (1907-1988)
American science-fiction writer
From the Urban Dictionary:
Usually applied to someone that has a character flaw or annoying habit they will never change because they simply aren't able to.
A person with bad hygiene - you tell them they stink but they don't really care.
"Getting you to clean your room is like teaching a pig to sing; it's a waste of my time and all it does is annoy you."