If you no longer require access to a company you can leave the company which will remove your access. When leaving a company you can't be the owner, so if you are the owner you will need to transfer ownership to another user prior to leaving.
To leave a company* follow the 2 steps below:
1. Select the 'Number of users' option on your dashboard followed by 'Manage users'
2. Select the 'Leave company' option shown against your user
*If you are a user on multiple companies you will need to leave each individually.