The first step is to click on Manage at the bottom left corner of SalesScreen.
A menu will pop up, where you can proceed to click on Users under the Company headline:

In the list of users, select those you would like to deactivate by clicking the checkbox on the left side and then click Deactivate User:

Once done, the deactivated users will appear under the Inactive User list:
​

If you wish to reactivate a user, follow the guide showing you how to reactivate a user.