You "can't let users add fields to tables"? I'm hoping you mis-spoke. Users should never be allowed to add fields to tables. This is, in effect, changing the design of the database. You lock everyone without an Administrator account out of "Manage Database" as well as Layout access. Perhaps you meant adding fields to Table View. In this case you deny access to Layout Mode. All this is managed through Privilege Sets. If you want to prevent scripts from doing things you don't want to allow, turn off (uncheck) "run script with full access privileges".
The issue is when a user changes to table view, they now can add fields in FM11 as well as records by clicking a plus sign on the layout. I have the menu set programed to turn this feature off. There may be a new setting in fm11 that I'm missing.
At this point I have upgraded our server to fms11 and my fma to fma11. All other users are on fm10.