This has been an annoyance of mine in phpbb forums for a while, and finally looked up today if there's a workaround. It looks rather easy to do, but may have to be done within each template too. Folks can always right click on a link but like me, I'm sure people don't think to do this every time. I think this will only work if you use the phpbb URL tag.
http://www.websitecodetutorials.com/cod ... window.php