The touchpad can be de-selected in SETTINGS....DEVICES....TOUCHPAD. My Windows 10 allows deselection of the touchpad when a mouse is connected. After reading the responses here, I tracked through some of the options in SETTINGS until I found the sequence above and deselected the pad. Seems to be working...or not working, as it may be!
Thanks for asking the same question that has bugged me for the past several years!