cordon sanitaire

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A barrier designed to prevent a disease or other undesirable condition from spreading.
• noun A chain of buffer states organized around a ideologically dangerous or potentially hostile nation.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A barrier (physical or administrative) to prevent the spread of disease.
• noun by extension Any barrier to the spread of anything deemed undesirable.