Limit

cross the line

Meaning: behave in a way that is not acceptable

Example: This newspaper has crossed the line. This article is so offensive to so many people. Read on

line in the sand

Meaning: a boundary beyond which one cannot proceed without consequences

Example: His resolve to either get a paid internship or drop out of school was his line in the sand. Read on

draw the line

Meaning: to define a limit in anything

Example: It all depends on your concept of fiction and where you draw the line between fact and fiction. Read on