Herb, here's another thought: have you seen that this is actually a true and written rule? Or just some park person's singular opinion or policy? Because if it's not truly a written rule, then .... I hate to say it .... but I would just avoid "just that one person", and choose a more opportune time (when he's not there).
Too many people assume, when one cop, or one gardener, or one city person says "scram", that it must be gospel truth law/rule. Not necessarily the case.
Example: one time, a few years ago, in my city, I had an out-of-town md'ing buddy visiting. I took him to a park that I'd detected here since the late 1970s. Mind you, I've never had a problem there. And to my knowledge, nothing official in our city's books. However, we just happened to be there when a park staff was working. He saw us across the park, and drove over to say "no detecting". I said "since when?" and told him I'd detected there for years, no problem. He said "
new policy, that his boss told him to "BOL" for, since they had someone else that left holes". Once I heard that rationale, my buddy and I gleefully showed him the area we were at, and I told him "
we just did 10 recoveries in this exact area, and as you can see, you can't even tell we were here" 
The park staff looked at the area, and had to agree, that we weren't the ones who were a problem. However, he said
"I'm just telling you what I was told" 
Then he said with a wink: "
When I get off work, what I don't see, doesn't bother me" When my friend heard that, he says "great, what time do you get off work?" The fellow said "5pm". It was 4:45pm then. So we merely went and sat on a picnic bench for 15 min. Sure enough, at 5pm, we saw him get in his truck, and leave. We merely resume detecting. That was 6 or 7 years ago, and I've been back to that park multiple times, and never heard a peep again.
I'm not suggesting this for places with an actual "rule" that specifically "said" no metal detectors. But if it's just an isolated booting, or just someone's morphing of other things in a "policy" type of interperetation, then no, I do not take this as a "rule". Same for cops bootings: You'd be surprised how often they are just responding to some busy-body's call in. And as such, they are duty-bound to make a decision. When in fact, perhaps they personally don't care. Again, if I know it's not an actual rule, I'll just give that one place a rest, and return at a more off-time.
Too many people take an isolated incident, and assume it's "the law". Not saying its' always like this. Nor am I saying to back and parade yourself in front of this individual the very next day, but .... just sayin' .....