In the responsibility, remove the dashboard from the list of avaialable tabs.
Then display any custom dashboards on a custom web applet.
However do note that the lookin selector is not available on custom web tabs.
In Step 4 of the role, i.e. privileges, there is a privilege called "*Access Analytics Dashboards*". Please remove this privilege and the users having the role would not be able to view Dashboards. This would remove the access to Dashboard even if the user's role has Dashboard tab enabled.
Alternatively you can remove the Dashboard Tab to remove the same.
However, I believe the best practice would be to remove the privilege and tab.
I hope this answeres your query.