Definition: To block entrances so that people cannot get into a room or a building.
Other definition:
1. To be separate from something that one would normally do.

E.g.1. The police sealed off the entrance to the library after the fire.
E.g.2. “We must seal off the room and make sure that no-one touches the body!” said the detective in charge of the case.

This phrasal verb can be separated.