So WPF windows only have four resize mode options: NoResize, CanMinimize, CanResize and CanResizeWithGrip. Unfortunately, the
You can create a custom window by setting WindowStyle to None, which removes the Minimize, Maximize and Close buttons, and create yourself the buttons you need. That's a great example for this:
http://www.codeproject.com/Articles/131515/WPF-Custom-Chrome-Library
It gives you some extra work, but if you realy don't want to use WinAPI, that's an option.