Even if a user doesn't have permission for a site in the wiki the user gets the page as search result. The problem is that some information of the restricted site can be seen in the search results.

Does anyone know how to prevent this.

Did you try Protector or Gatekeeper already? They might be able to hide the restricted content like you want. As far as I know they don't hide cached content though, so would also have to prevent caching of search results (and modules). You could also use the Groups Package to create content that can only be accessed by a certain group.