Shoutbox

It's a poster.

So my initial thought was right

You should be using powerpoint for a poster.  It's as simple as that.

If you want to use word then it looks like you're left with the option of either fiddling with the settings for all eternity, or printing it out as close to borderless as you can get it and then cutting the rest out yourself.

Or you could just make it easy for yourself and use powerpoint

It's your choice