I have a Satellite receiver that pickes up a few channels from overseas. I put a 1x4 splitter on its output to route this signal into 4 rooms. The standard television sets in other rooms handle this signal perfectly. But one room has a "TV BOX" used for routing a television signal to a computer monitor, in this case a flat screen LCD. This line seems to return a rogue signal to the satellite receiver that somehow messes up the master signal going out from from the receiver, resulting in all 4 split legs performing poorly. Can I put an rf isolater or circulator on the offending line to improve overall system performance? Recommendations?

I would try to an attenuator first. Try a 3 or 6 dB. Or an amplifier and attenuators.

If you have an isolator that work at those frequencies of course you can try that too.

Be sure to terminate the unused ports.

If your coming out of the receiver then you are low frequencies. You can try placing an amplifier on the line (Radio shack cells them) to isolate the path. Most amplifiers will gve you as much if not better isolation than an isolator.