Unfortunately there is no one click solution or process to remove Company Access for all users, however, there is a way to optimise removing access to all users that can get into that Company.
This can be done in the following way:
Go to System Admin
User Setup
Then select any user and click the Company tab at the tab
Select the Properties option
In here use the Filter (CTRL+F) and Filter to the Company Code and clear the User ID field. This will show all User's with access to the Company currently in one list.
You can then use the Right Click and Delete Record option and run through the list one user at a time.