Security trimming should prevent users who do not have permission to view content from seeing that in search results. The results are filtered at the time of the query - however the access rights might not be up-to-date in the property database until the next crawl.
If you want to quickly remove content from search for any reason, go to the Search Administration page (Central Administration->Appliation Management->Manage service applications->Search service application).
On the left-hand column go to the Search Result Removal link under Queries and Results. This will allow you to add URLs to immediately remove the offending content from the index and also add a rule to prevent it being added to the index in future crawls.
This will affect all users, of course. You could then remove the crawl rule again and start an incremental crawl. That should bring it back into the index but with updated properties so that it would not be visible to the forbidden group.