Yes you can do this.
Go to your Gossamer Forum admin:
1. Setup > Actions
2. Select user_list (Viewing Member Directory) from Edit action drop down
3. Then edit action options you want
You can hide the action from shown on "Who's Online", and also set minimum user permissions for this action to minimum user status of an Administrator only if you want to use this at any case or simple disable that action.
Hope this helps
Cheers,
Boris
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