I mean, in theory, you could do an ACL, something like this.
Blocking IP (L3) between clients, but permitting the L3 ports used by Webex to "any" aka, between clients, and to the rest of the network (and internet) on that SSID.
This does of course NOT block L2 traffic, but it should (will) block L3 traffic.
So if you have security concerns about pure L2 traffic, of course this will not solve your problem.
PS: above ports used are taken from the webex documentation.
I have no idea if these are the ports used for traffic (RTP) between clients. 🙂