Signs are great, awareness is even better. Don't trust a sign to accomplish all you need to do to maintain access. Drafting and discussion a SHORT list of essential rules for your launch directed at the key issues is a good first step. Actively promoting the rules among kiters, discussions with authorities to show you are concerned and are working towards some solutions can help a lot. Always show patience and politeness to bystanders, even if they will continually stand in poor locations. There have been plenty of cases in the past where bystanders after receiving fair treatment and explanations, go out of their way to spread the word to other bystanders.
Some kiteboarding launch area signs. Click on images for full size depictions. You can print these out in 11 x 17 inch format, laminate them and instant sign. Or, just have a sign maker prepare one. You can't always get permission to erect signs at launches all that easily. As an alternative, you can put them up on foldable A-Frames (out of PVC or ... ). They can be put up and taken down each day. It would be nice if the sign won't readily tear kites either if caught by accident.
Some examples follow. Click on images for full size depictions.
HKA sign at kitebeach on Maui, HI
Sign at Matheson Hammock, Miami, FL[/color]
Sign developed in Pompano Beach, FL
Sign developed by Mark Rush in Florida's Panhandle
A newer sign from Ireland
Sign developed in Ireland
Sign developed in Essex, England
CNK kiting rules of the road sign developed in France
IKO Priority Rules