With the solo thing I find that the term "stage" has been changed to mean a lot of things. Only one place I now play has a real stage - the others don't. With a real stage it's fairly easy to ask people not to walk onto the stage... if you have to explain why then tell them because of all the cords and the potential liability for injury.

However there is one hard rule I stand by - no drinks near my gear!!! I had to enforce this just last week when an older gentleman came towards me with a drink in his hand and was obviously going to lean over the front of my keyboard to make a request while I was playing. I stopped the song I was playing immediately and stepped around the keyboard to lead him back a few steps and told him politely not to get near my equipment with a drink in his hand. The fact that I stopped a song in the middle alerted the management also and they came to see what was up. The man was startled but understood he'd made a mistake.