You can remove a member from your account completely from your app settings (this is especially helpful if someone has left your company and you can't remember which collections and workspaces they're a member of!).
To do this, click on your avatar in the top right hand corner, then on "Subscription".
If you're on the free plan, you will need to click on "Upgrade" then "Show usage". From there, click on "Details" next to "Members".
All the members in your account will be listed, including which workspaces and collections they're members or even owners of (marked in bold). To remove them completely from everywhere at once, click on "Remove" next to their name.
Note: If they are the owner of a collection in a workspace owned by you, you will get an error. Please ask them to transfer ownership of the collections they own to you or someone on your team.