To achieve similar functionality to Cisco's private VLANS, where all ports are on the same L2 segment, but cannot exchange packets, you can use Mikrotik's Bridge Horizon feature.
Basically, every port in a bridge is assigned a horizon value, and RouterOS will only forward frames to other interfaces in the bridge that have different horizon values.
This means that you assign the same horizon value to the interfaces that you don't want to be able to communicate.
For example, you want to bridge all your customers and use a single /24 subnet and the same gateway. Typically this is bad and poses a huge security risk, not to mention performance issues. If you assign the same horizon value to the customer interfaces, then the router will not forward traffic between customers. Customer A will not be able to ping Customer B.
If you had a server, such as an IP-PBX that all customers needed to access, and you were lazy and added it to the bridge, then you would assign a different horizon value to that port.