is this a city level park? county level? state level? fed. level? etc..... Are there any actual rules which *specifically* say "no metal detecting" in that cities (or counties, or state, or whatever) laws/rules? If not, then the fellow had no reason to boot you. Unless, of course, he says "he was making a mess" (even if it's not true, if you had even so much as a PROBE/SCREWDRIVER to poke around with), and you'll "loose the debate". So sometimes it's just better to steer clear of these persons, give lip service, move on, and just return when they're not on -shift. Ie.: low-traffic off-times.
It's actually gotten to where I simply do most of my park -hunting at dusk, or early AM's, or any other odd-ball times, JUST to avoid busy-bodies. Not because there's necessarily any "rules" (there rarely is), but to avoid any connotations, which, admittedly, this hobby has.