Adding or Removing a paid Salesforce license from a user is covered in this help article.

But what if you want to delete the user entirely?

The list of Salesbolt Users you see in the Billing tab is the list of users in your Salesforce Org. So if a user is in your Salesforce Org, they can and will appear here.

So how do you delete them? Well, you can't delete a user in Salesforce. The functionality Salesforce provides is to make the user inactive. And simply making the user inactive in Salesforce will prevent them from using Salesbolt.