Why not take the view that the person who has flown with the PPL might just get enthused enough to come to you as a student? If they want to learn to fly, then they'll have to do a proper course; that's where you come in. Don't get jealous about other people showing them how it works. Just ask your new students if they've ever flown an a/c before. If they say 'yes', just be aware that you'll probably have to 'un-teach' what they did before and get them in to good habits. Bite your tongue when you hear of other people doing things, treat them as potential customers.
Above all, enjoy your instructing, it's hard work but can be very rewarding (but not financially!)
TOO