Mamoth by the sound of it this is just a bit of a park - the women's sessions and over 6 5s sessions etc would be staffed and policed because they would be in a swimming pool or gym not an unstaffed public area. If there was a man at the women's session or a young family at the over 6 5s session a staff member would ask them to leave - I've been asked to leave because of blimin water yoga or some such thing I hadn't noticed the signs for (nor would I have expected the entire pool to be closed for it if I had, but it was) and I wasn't the only one.
Most limited access sessions or events have some form of staffing or announcements otherwise people going about their business with no intention to intrude often are just oblivious.
Greenolives very few people check the council website before going to the park - a lot of people go spontaneously or every day, in either case they don't check the website first in general. You might but most people go about their days without checking the website of everywhere they might go before every visit - it's not an obligation, and websites are anyway often not kept updated.
Where there is no staffing or announcements people will unwittingly be in the wrong place at the wrong time regardless of the odd sign, and that does apply to all types of restrictions not only SN sessions.
Some kind of staffing at least to make announcements or close the area 15 mins before then reopen to the target group (with an announcement not by interrogating people) would solve the problem or at least reduce it to the few who genuinely were deliberately brazenly misusing the session!