terminating alimony in wisconsin

You don't indicate if the maintenance award was based on an antenuptial agreement ("prenup") or was determined by the court. If there was no prenup that set the maintenance amount and there has been a substantial change in circumstances, you can petition the court for a modification of maintenance. The fact that she is living with her boyfriend and her expenses have changed may suffice as a substantial change in circumstances.

You should consult with a family law attorney to help you with this - the facts here are pretty sketchy and your attorney will need more information to put the facts together in a way to support your position that there has been a change in circumstances and it would be appropriate to reduce the maintenance owed.
Good luck - please contact a family law attorney in your area.