cordon

A cordon is a line drawn round a scene to keep people out. Tape, barriers, or officers standing shoulder to shoulder.

Police work is where the word lives: a police cordon, throwing a cordon round the building, and the phrasal verb cordon off, which is far commoner than the noun. A cordon sanitaire is a barrier meant to stop something spreading, whether disease or political influence. There is also cordon bleu, the blue ribbon of French cookery, which shares an ancestor with the barrier and nothing else.