Currently, there is no way to permanently remove users from Chatavise. Instead, you can set old users to inactive on their profile page.
You can do this by going to the Users page and searching for the user you want to set as inactive.
Click on their profile, and at the bottom, deselect the “active” checkbox. Hit save, and that user will be marked as inactive.
This will remove the user from your Chatavise users list visually, but if you ever need to find their information again and readd them as a user, just search their name on the Users page!