One caution...assuming it is threaded, it's probably on a threaded nipple. The other end may unscrew when trying to remove it...this would mean it's now leaking into the wall. As Terry suggested, if the flow isn't too great, and there's any elevation differences in the units, if you can open those that are lower, you may drain off enough to prevent water from getting to that unit until the main is turned back on. Tough to do while you may have numerous people wanting the water on, though.
There is a tool designed to install a new shutoff while under pressure. If you really can't shut the whole system down, you might want to bite the bullet to have one of those installed. I supposed you could do it yourself. I don't remember the name of the thing, but it basically starts by finding a suitable location. The pipe needs to be clean on the outside. You clamp the sleeved body of the thing around the pipe (internal gaskets make that water tight), then the cutter goes through an opening on the top of the sleeve. It has o-rings to prevent water leaking as you cut through. As you continue to go through, the valve mechanism is attached to the cutter and slides in place, then you anchor it in place from the bottom of the new hole you made through the pipe on the other side of the sleeve, and you're done. Probably cheaper than paying a plumber to come to your island. You may be able to then install building shutoffs that work as needed, since the main now can be shut off, and further isolate things so as to not inconvenience the guests, or if you can get to the supply to each building, do those as needed until you have them all. Since this valve is more expensive, it might be better to do the complex main, then use regular ones on the individual branches to the buildings, if possible, but that would be up to you to decide what is more cost effective verses the inconvenience.