Here's an explanation from Blizzard:
(
http://www.battle.net/scc/faq/trouble.shtml)
QUOTE(Blizzard Entertainment @ "SCC: Frequently Asked Questions")
Why can't I remove specific Map Revealers on my map?
Patch 1.05 for Brood War and StarCraft will allow you to remove all map revealers on a map, but not individual revealers within a location. Invisible or hidden units are not technically findable by trigger, just as you cannot find units in a transport, which are also marked as hidden. This means that you are still not be able to remove any Map Revealers from a specific location, since that would require the game to find them within those specific coordinates, which it cannot. If you need to remove specific revealers, the way to do it is to remove them all and then recreate those that should remain.
Instead of using a map revealer, try using a player 12 burrowed unit or observer (as others have suggested). I know at least that if you create a burrowed unit for a player and then give it to player 12 using a trigger, no-one will be able to see it. But I suspect that you can simply preplace the unit and it will still work.
Addendum:I tested it.
Preplaced burrowed or cloaked p12 units are invisible to players without detection.
Burrowed or cloaked units for players in-game given to p12 are invisible to players without detection.
I suspect that p12 units are only revealed if they're given to p12 as a result of a player's defeat.