I need to turn automatic margins off according the following statement from Screen's manual in my Mac
If your terminal is a "true" auto-margin terminal (it doesn't allow the last position on the screen to be updated without scrolling the screen) consider using a version of your terminal's termcap that has automatic margins turned off.
How can you turn automatic margins off by your terminal's termcap?
If I understand you correctly you're looking to set the autowrap feature to NO using terminfo database. If so I believe you can use the -nam flag to turn it off - something like vt100-nam should do it. You can also check by looking at the man pages for terminfo.
If this solves your question, mark this up. (^_^) If not... well comment back and I'll check again for you. Cheers!
Update: There's also a shortcut that may apply to you to toggle the wrap off and on. Check out the shortcut sheet here. And additional information for Screen can be found here (search for wrap). You can also check here on how to use setterm (section 17.14 Changing the Terminal Settings). Also check here for examples of changing settings.
Good luck again. (^_^)