To exit split-screen on a Galaxy S11 Ultra tablet, drag the divider line between the two apps all the way to the edge of the screen to close one app, or tap the Recents button, select an app's icon from the top bar, and tap the X to close it and exit the split screen. To prevent accidental split screens, turn off the "Swipe for split screen" and "Swipe for pop-up view" options in Settings > Advanced features > Multi window.
Also can you give more details as from which apps is this most annoying? This is a feature that can come handy sometimes.
