Dic: draw a line in the sand

• The government is trying to draw a line in the sand regarding public sector pay rises.
• Todd drew a line in the sand by giving his roommate an ultimatum about his sloppiness—he had to start cleaning up after himself or move out.
• The president has drawn a line in the sand, which means that if the foreign troops are not removed, they will be attacked

1. set a limit that you refuse to allow sth to go beyond
2. Fig. to create or declare an artificial boundary and imply that crossing it will cause trouble.
3. to say that a particular idea or activity will not be supported or accepted .

.....OALD, http://idioms.thefreedictionary.com/Sand