When I saw a message that had content completely irrelevant to the thread, I reported it to the moderators like one normally would. Once I sent it, I get a notice that the message has been sent successfully. So far, this is what I expect to see. Included in that message is a link that states "back to the previous page" or the like. If I click it, I only get an error stating that the message has already been sent. There's no other way to return to the forum outside either using the back button a few times or closing the window/tab and reopening it. On my 0 to 8 severity scale, this ranks as a 3, moderately minor.
The report link opens a new tab/window, so replacing the function with history.go(-2) (to go back two pages) makes no sense. That tab or window only has one page in its history. IMO, the link should call window.close(). Once you've reported a post the tab is unneeded, so it might as well be closed.