FYI, tampering with mail is against the law. (So is property damage, for that matter.) Call the police, report the situation; I rather think the above idea about having the area under video surveillance isn't a bad idea, especially if this continues happening. For sure make the report, though, so that if something more severe happens in the future, you'll at least have the initial complaint on the record.
If you are sure that your mail is being tampered with, you can have the post office hold your mail for pickup. Annoying and not the least bit convenient, but you also don't want to miss any important bills or the like.