Below is a quick way to limit only certain users the ability to SPD.
-Out-of-the-box, users who have Full Control and Contribute security permissions can open SPD and make changes.
-Assuming you want only users with Full Control access to have access to SPD, the easiest way is to turn off access to the site from SPD for users under the Contribute permission level.
-Open the site
-Select Site Actions > Site Settings.
-Under the “Users and Permissions” group, click the “Advanced permissions” link.
-Select Settings > Permission Levels from the toolbar.
-Click on the Contribute link.
-Under Site Permissions, uncheck the “Use Remote Interfaces - Use SOAP, Web DAV, or SharePoint Designer interfaces to access the Web site.” option.
-Click OK
The issue with the above steps is now any user with Contribute access cannot use SPD. You may have some users who you still want access to SPD but do not want to give Full Control to. For that scenario, you could create a new permission level.
-Open the site
-Select Site Actions > Site Settings.
-Under the “Users and Permissions” group, click the “Advanced permissions” link.
-Select Settings > Permission Levels from the toolbar.
-Click on the Contribute link.
-Scroll to the bottom and click the “Copy Permission Level” button.
-For the Name, type in “Contribute (With SPD)” (without the quotes).
-For the Description, type in “Can view, add, update, and delete. With SPD support.” (without the quotes).
-Under Site Permissions, check the “Use Remote Interfaces - Use SOAP, Web DAV, or SharePoint Designer interfaces to access the Web site.” option.
-Click Create
Now the out-of-the-box Contribute permission level can contain all the users who need contribute access but no SPD. For those users you want to give SPD access to, they can be added to the “Contribute (With SPD)” group. One caveat is if you have other applications access the SharePoint web services for the site, they will no longer work.