Isn't there a trigger action to unselect a unit? have a look under the "selection" category of trigger actions and see if there is. If you use that, it would mean that every time a player selected a unit, the unit would become instantly unselected.
Of course if you have a spare player slot, a better way to do this would be to make the units owned by another player and disable unit control for that player. But this will only work if you have player slots to spare....
oo, I just realised maybe another better way to do it is to just give all your units the locust ability, making them unselectable in the first place! Of course this may not work, because I think someone once said giving a unit the locust ability also makes them invulnerable....
