@matthijs_ 

ACL will stop the talk even you don't enable bi-directional. Deny A to B will stop the talk between them. There is no way you can do with this even you permit A to B before the Deny rule. I don't see a way to have the unidirectional talk across VLAN. 

@John1234 

Thank you for your response.

That's too bad. We regularly use this functionality on Watchguard Firebox and Netgate pfSense.

Since when creating a rule you are asked to specify a source and destination I assumed that it would only be blocked that way.

@Fae

Are you able to confirm whether or not unidirectional talk across VLAN should be possible?

Thank you.

In my experience, at least in standalone mode, it doesn't not block bi-directionally.

When I wanted to stop any traffic between two vlans to both directions, I needed to create two ACL rules for each.

  @Arion 

Yes, that's what I would expect. On the ER605 router?

Via another channel, tp-link indicated that this functionality has been intentionally disabled to save processing power on the cheaper models. A solution that was suggested to place a tp-link switch with acl functionality behind the ER605.