In this particular case documentation will be important.
This documentation should include when the warning was issued, was it verbal or written, what are the consequences and what plan, if any, will there be to help the offender mend his ways.
You can give a verbal warning followed by a written warning then dismissal. Or you could go verbal then dismissal. Or written then dismissal. The more complete the doc trail the better prepared you will be for a wrongful discharge.
You also want to be careful because by allowing it to continue unabated you are developing a culture of acceptance of such behavior. That can count against you should you have an upset ex-employee.
More importantly though you might want to find the answer to why they are damaging property in the first place.
Are they simply careless? Or could they care less? Is it the same kind of damage every time or do they find new ways of breaking things?
I've got a climber that I just can't let work over fences. Just like the golfer that always hits a pond no matter how much they don't want to, this climber seems to zero in on the fences.
He doesn't want to, he doesn't try to, but it seems the more he attempts to not do it, the more accurate his devastation is.
Maybe there is a brain-block there that makes them do the stupid dance. We all have something like that that we must guard against.
Anyhoo, how long do you let it go on? Well, how much in repairs are you willing to pay for? Are you willing to see someone injured or killed before you send the offender packing.
Try being proactive...send out a memo to everyone and include it in their employee files. Let them know that such behaviour cant' continue and if it does then its time for a-packing.
It is your business, your future and your reputation at stake and should defend it with everything you have.
Anyhoo... just my two cents...
(excuse me for a moment, I need to get the soap that fell out of the box I've been standing on
)