Thanks for your interest in Aspose.Slides.
I regret to share that there is no such option to define the text cut off the text frame. There are two properties of TextFrame namely FitTextToShape (
http://docs.aspose.com/display/slidesnet/FitTextToShape+Method) and FitShapeToText(
http://docs.aspose.com/display/slidesnet/FitShapeToText+Property) that can be used to accommodate the text inside the text frame. Please visit this thread link for your kind reference as well. You may need to develop your own logic to define that whether text will run of the TextFrame or not.
We are sorry for your inconvenience,