I'd have applauded you. On one occasion (during the opening weekend of one of the Harry Potters - I forget which) a group of giggling adolescents were annoying everyone during the first five minutes. Then a big, angry-looking South African man got up from his seat, strode over to where they were sitting and warned them in no uncertain terms that if they didn't shut up, it would be the last thing they ever did. Everyone else in the cinema applauded him, and they were silent after that.

Therefore every cinema needs either an angry South African, or an equally angry (former) Glaswegian. :)