There are a few ways to keep Triss without romancing her. One way is to simply tell her that you don't want anything serious and just want to stay friends. You can also lie to her about already having a girlfriend or wife. Another way is to make sure you never do any of the romantic actions with her, such as giving her flowers or kissing her. Finally, if you have Geralt ask Yennefer to help him track down Triss, this will allow Geralt and Triss to part on good terms without romance.