You can remove the subscriptions and deactivate the users in Core from the Manage Users screen, which can be accessed from your company menu > Account > Manage Users. Typically, the Administrator or Owner of the Core account has the permissions to do so.
To remove the subscriptions, you must click the Package column of that user and remove the subscription package. To deactivate the user, select Inactive under the Status column of that user. Click Done to save the changes. You can also deactivate users by selecting them in the grid and clicking Actions > Make Inactive. Check the Core Help for details.