SharePoint Designer is a free tool available from Microsoft that allows for more advanced development of SharePoint sites. However in some cases, clients have asked to disable SharePoint Designer access for Site Owners and Designers (Note: Site Collection Administrators will always be able to edit in SharePoint Designer). The good news is that this is an easy modification to make.
First, let's look at the current Site Actions drop down menu on my Office 365 SharePoint site:
Notice how Edit in SharePoint Designer is an option.
To remove this an an option, select Site Actions and then on the drop down select Site Settings:
Select SharePoint Designer Settings and the following options appear:
Uncheck the Allow Site Owners and Designers to use SharePoint Designer in this Site Collection. There are several other customizations regarding SharePoint Designer development that can be set here as well. For more information on these settings click here.
Now, the SharePoint Designer option will no longer appear in the Site Actions drop down list for Site Owners and Designers.