What kind of component would I need to put a break in a RS-485 bus?
You can just use a relay, optionally switching over to a couple of termination resistors each side.

Edit following WRB's comment: If the system can't stand a bit of junk on the bus (which would seem like an oversight in something relying on RS485) then look at quiet relays (mercury wetted for example) or perhaps putting two RS485 transceiver IC's back to back.