Is there a way...

[Ualaa]

They removed the ability to check if a buff is up or not, via a macro or script.
The ! command is saying do this once, but no more.

So /cast Stealth, on a macro could put you into stealth and then pull you out, if spammed.
But /cast !Stealth will only run it once.

However, I'd just go with:
/cast [nostealth] Stealth

Or my Paladin has:
/cast [nomounted] Concentration Aura
/cast [mounted] Crusader Aura

My druids have:
/cast [nostance: 3] Cat Form
/cast [nostealth] Prowl