Good form in cases like this (such as excessive noise, which I know this isn't, but the point remains) is to be co-operative, and not at all confrontational. Telling someone you've reported them to the authorities without giving them a chance to defend their position does not count as such, but is something worthy of Lynda Snell. Much better to warn / tell the person / people in question first that something bad has happened and ask them if they know anything about it, rather than snitch in the first instance. Indeed, not doing so immediately disproportionately strengthens your case should they reoffend, and *then* you have the moral right to escalate. (cue 10-page thread...)