To “unthread” a text frame, ie. stop the text flowing into the next text frame, double click either the out port where you want to make the break or the in port of the following text frame.
Amazingly, there is no way to break the connection between two text frames but leave the content: You have to cut and paste the remaining text into a new frame. However, one helpful thing is that if you press Ctrl (cmd) + shift + end it will select all the text from that point on, then just cut and paste.
If none of this works, and the text frames stay threaded, try turning off “Smart Text Reflow” (智能文本重排) under Preferences > Text.