Disable Split-Screen/Multi-Window for apps
[Is] there a way to simply change what occurs when the recent app button is held?


You can tweak the Overview (Recent) button for long-press event using GravityBox [N]. GravityBox requires a comapatible Xposed Framework. 

In GravityBox module, go to Navigation keys actions &rarr; scroll to Recents key &rarr; tap Long-press action &rarr; choose an appropriate action. 

From then on, long-pressing the Overview button would not show spit-screen mode. 

Tested on Nexus 6 running stock Android 7.1.1.