Drawing a line in the sand
There are two real problems with this attitude:
First, drawing lines. Problems aren't linear, people don't fit into boxes. Lines are not nuanced, flexible or particularly well-informed. A line is a shortcut, a lazy way to deal with a problem you don't care enough about to truly understand.
Most of all, drawing a line invites the other person to cross it.
Second, the sand. Sand? Really? If you're going to draw a line, if you're truly willing to go to battle, you can do better than sand.