When people leaves the company i always put the user on a deny access group, remove the account and delete the mail database.
But if you do not delete the account…
Once the user added in Deny access group to Deny Traveler access you need to enable “Enforce server access setting” to Yes in the server document under Ports -> Internet Ports -> Web tab.
Follow the below steps to enable the ‘Enforce server access setting’ to Yes in Server document:
1) Open the Domino Administrator client.
2) Chose the Traveler Server document .
3) Go to Ports -> Internet Ports -> Web tab and set the value for the below field to “YES”.
Enforce server access setting
4) Restart the HTTP task to take affect the changes
Found the information above on the TN 1634205