One way we can restrict access to SharePoint Designer is by creating a custom permissions level on the specific SharePoint site collection:
I’ve ran a quick test with a contributor group called ‘SPDesigners’
1. In your SharePoint Site, create a new permissions level (called ‘SPDesigners’ in this example) and assign your unique permissions to this group
a. Open the SharePoint site in the browser.
b. On the Site Actions menu, click Site Settings.
c. On the Site Settings page, under Users and Permissions, click Advanced permissions.
d. On the Settings menu, click Permission Levels.
e. Then set the permissions you would like the permissions level to have, here’s my attempt:
f. Click Create
2. In SharePoint Designer, create a new Contributor Group
3. Click Site > Contributor Settings
4. Give you new permissions group a name and link your permissions level to the contributor group:
· On the left column, set specific access levels to allow / disable folder create, sub sites, page editing, formatting (ie. Css), image management, workflows, block overwrites to existing pages etc as desired.
· Click OK.
(Further information : http://office.microsoft.com/en-us/sharepointdesigner/HA101009091033.aspx )