Possibility to exclude terms

Sometimes you wanna have a construct like

"
|some_thing
|some_other_thing
|browser
|----ie
|----ff
"
and you wanna let the users post into the categorys ie and ff but not directly into browser.

Since I don`t know a way to tell drupal to let users not post into a special term (vocab?) like bere browser - it would be nice if your module have the opportunity to exclude some terms. 99% of the users choose terms from the "suggested tab", so this really would "solve" that problem.

Thanks, sincerally l8a