When an employee leaves the company, you'll need to remove them from Office 365. Before doing so, you should block them from accessing company files, preserve the documents they created, and perform several other admin tasks associated with removing a user. 

For more tips like this, check out the working remotely playlist at www.youtube.com/FoetronAcademy . Also, if you need any further assistance then you can raise a support ticket and get it addressed.