Ive been try to solve the same problem you have. I have an ethernet hotspot gateway connected to an external AP.
I have tried to do the router IP isolation as suggested above (!10.5.50.1/32) but the clients can still access each other.
I have tried the following solution, but still needs further testing and packet monitoring.
Ive placed additional rules in /ip firewall rule forward, drop all traffic from UDP port 137 - 138 and TCP port 139. Same rules in /ip firewall hotspot-temp.
If any of you guys has a better way of doing this please post your solution.