i think there's no other solution.. blzzard needs to add this in a new patch, so that you can adjust this.. or you use some kinda custom HP-Bar, so make it invulnerable so no Hp will be shown, as i know it's possible to attack invulnerable things, but doesn't kill them or something like that, however, you can still save the damage done, and link it to your custom Hp-bar, if the life gets 0 you make it mortal and kill it or somethinglike that..
But i would suggest to simply reduce the selection size, because it doesn't effect the selection of units in any case, it's just the display of the selection circle, if it's smaller you wont see it for the building, but you could add some special effects on it, when something is selected if you want to see some effects.. but yea.. reduce selection size, that's the best and only way as i know