Prohibition to the designer

1. Remove the 0-line on forms from the "ALL" role.
2. Add every form except that specific one for exection rights in the all role (this is quite easy with the "all objects" button).
3. Create a new role and put the execute permission on that specific form
4. Add that new role for that specific user.

... something like that ...

Must crude way is putting the following code:

IF USERID <> 'BLAH' then
  error;

effective though