In the Verba Ethical Wall solution, Communication Policies include Disclaimer functionality.
You can set Disclaimer Templates to be attached to all Instant Message communications of various parties.
In order to create a presence blocking policy:
Step 1 - Create a new Communication Policy
Step 2 - Configure the Participants A and Participant B groups
Step 3 - Configure the Disclaimers category
Step 4 - Press Save
You can mix Presence Blocking with Presence Policies, Session Policies and Content Polices on the same Communication Policy.