In the 6 years I've been cycling through the park, I think I can genuinely say I've only seen about 4 reckless, speeding cyclists. When the signs went up, I was quite surprised as I'd never noticed there being a problem. I'd have thought the number of people letting their dogs mess the place up, and the teenage drinkers, were more of a concern as that's something I do see frequently.
However, that's digressing. Slowing down bikes? Speed bumps across the width of the path would work, though they'd become pretty slippery & lethal to cyclists and pedestrians in winter / wet weather. Or you could put a fence up the middle of the path and have one side for cyclists and the other for everyone else? I don't know - it's a hard one. Maybe just employ the torch-woman to be on permanent patrol (although she shone it in your face whether you were barely going fast enough to remain upright or not!)