Then DO WHATEVER YOU CAN to break the affair. This is of UTMOST importance.
1. Cancel the cell phone if it's in your name 2. If it's not then make sure the family's money is not being used to support the affair.
I can't remember if it was you whose W refused to move out or not... if it wasn't then do as RobX says... pack her bags and kick her out. Even if it is illegal... most of them don't know this and by the time they do the psychological impact and lesson has been set.
You DO NOT allow her back under any circumstances for AT LEAST a month of her whining, begging and pleading.
You have to show her that you have got BALLS for her to RESPECT you. Once you have her respect you the rest will come slowly. Don't be a fool and fall for immediate (read as false) sorrow. She has to pay the consequences.
I've only seen the NICE GUY / hands off approach work ONCE so far in this forum -- and that guy had the patience of Job. I think he waited two or more years for his W to come to her senses. The other successes that have achieved turnaround have been through variations of tough love.
Tough love works for WAW's because it's the quickest way to get the W to respect you again. Note: Depending on how far gone your W is... even tough love may not do it.
