I don't believe the model flyer was breaking any law / rules as the 400ft rule only applies to models weighing over 7kg. That's a fairly hefty chunk of model aircraft.
It's a fair bet that the model flyer is an aircraft enthusiast or at least has an unhealthy interest in them. In this instance I reckon the best course of action (to start with) would be to befriend and educate the guy concerned.
We have a model a/c club who operate on our airfield, sometimes whilst we are flying. We work with them and get along just fine. Never had an incident in the fifty plus years they have been operating.