Restricting Access to Office 365 by IP address

Hi, 

We have a small private business that does not have an Active Directory domain. The users have their own PCs which they use to connect to Office 365 using Outlook. The head administrator on the site has discovered that they can access their email from ANYWHERE using the portal.office.com portal and does not want this. 

They have a static IP address on their broadband line. Is there any way that we can restrict Office 365 access to just the IP address of their workplace, so that the staff can only access their email at the workplace. 

thanks, 

J