When your users press the Sites button in Office 365 they will be taken to a list of SharePoint sites they can access. However, as you can see from the above they also, by default, have the option to create a New Site.
You may not want all users to have this option since it can result in many, many unwanted and unused SharePoint site in your environment. To remove this option, login as a global administrator to the Office 365 Administration Portal.
Select Admin then SharePoint from the menu that appears.
From the bottom of the list on the left select Settings.
Scroll down the left hand side until you find the heading Start a Site. Then select Hide the Link on the right. Scroll down to the bottom of the page and select OK to apply the changes.
Now when users view that same initial page they will no longer see the option New Site.