How do I disable split screen?

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!

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