My helmet, a Bell also, has a little, round, black rubber ring on the chin strap -- adjust everything once, then slide it right up to the buckle, and nothing moves. Of course, after almost four years of use, it finally broke a couple of weeks ago. But, I've not had any issues with straps loosening since then, so perhaps I'm just lucky. In any event, I use small silicon ponytail holders to achieve the same effect when helping kids with helmet adjustments/fittings.
"If we know where we want to go, then even a stony road is bearable." ~~ Horst Koehler