How do I disable split screen?

Presumably your iPad is running iPadOS15.

Unlike iPadOS14 and earlier versions, multi-tasking cannot now be disabled from settings. Multi-tasking has received a significant overhaul, both simplifying its use and introducing new controls.

Key to accessing and controlling (or avoiding) multi-tasking is the small “three-dot” button that is found at the top-centre edge of the screen.

More information about using multi-tasking can be found in this support page:

Use multitasking on your iPad - Apple Support

The iPad User Guide is also a rich source of information about using iPad and its many features. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:

iPad User Guide - Apple Support

https://books.apple.com/book/id1567104892

You don't.

Split screen/slide over screen multitasking is an integral part of iPadOS 15.

You can avoid split screen/slide over screen multitasking simply by not touching/tapping the three dots at the top of the iPad;s screen or within the top of the screen in any app.

By avoiding touching/tapping these three dots, this will make sure split screen or slide over screen views never appear.

Best of Luck to You!