Cleanse already works on knock-ups. However, it does not stop you from continuing the rising and falling of the animation but it allows you to use your displacement abilities to cancel this. Well trained LCS players often will cleanse-flash many knock-ups if it is beneficial to them.
The only alternatives to this are:
- Return them to the ground immediately
- Make cleanse produce a spell shield with a small window.
I really do not see any other way to do this. With banshee's veil, edge of night, and mercurial scimitar, I think this is unnecessary. CC being useful is essential for the health of the game so that squishy champions do not become completely dominated by a glass cannon meta. If CC was too easy to block, then squishy champions with low mobility (aka many ADCs) will have less ways to counter this kind of gameplay.