I get the jumbled tabs and extra spaces, too. This is the only thing I've found that works (for me & my operating system). It isn't quite as easy as copy and paste, but at least it gets the job done. See if the following works for you.
1) Use the Phorum's composition tool, from any window used for drafting a message whether it be "new topic" or "reply" or etc.,
2) Click the "Formatted code" button, bracketed code and end of code appear,
3) paste from Word document in between code and end of code,
4) select all, copy and paste to the website,
and
VIOLA!! It's there and perfect! (except there is the visible bracketted code at the top and the bottom of the body of the tab,
BUT THATS FINE!!!)
Hope this helps. I know the frustration you're going through.