Does the game have a built-in safeguard from spot-spamming (pressing the spot button repeatedly, while aiming in suspicious/distant/smoke-filled locations)?
It does, however I'm having a hard time finding any official source on it. Other than that all I can offer for proof is first-hand experience and several forums where it is suggested. It seems that you can only spot about once per second. Also if you press the spot button several times within a second, it'll require a few second cooldown time before you can spot again.
I did find a forum thread with a thorough explanation of the spotting function, however it still doesn't relate the information from anywhere official. From my experience it looks correct.
I do only play hardcore so things may be different on normal servers:
It has spam prevention. Just try it out by pressing "Q" very often repeatedly until it makes a sound (similar to the one in Half-Life 2 when trying to use something not usable ;-) - it's not very loud, though).
After that there is a cooldown until you can spot again. I'm sorry I don't know the exact numbers for times until prevention is active and cooldown. But it's definitely more than 1 allowed per second.
I think there is a safe guard against it, but the delay is so small, it doesn't really make that big of a difference. Many players use a macro that auto presses the Q button, making their jobs a little easier.