You shouldn’t be checking if the player has changed their walkspeed, but rather checking if they’ve moved too fast or otherwise done something that is impossible.
Another important thing to note is to never auto-ban people for exploiting; you can kick but I’d recommend against that too. The best anticheats just prevent a exploiter’s cheats from working entirely (like the server taking network ownership of their character and teleporting them back to a previous location if they try to teleport) rather than punish the player for potential false positives.
Here’s a really good anticheat that I’ve seen that simply tries to stop what an exploiter is doing rather than kick or ban them from the game, although it’s experimental. I’ve tested it out and it works really well and you may be able to take some ideas from this anticheat.
You could also check by having the server and client cross calculate their movement, and if the server ends up desynched you can move the player to the correct location, however that’s more difficult to achieve without writing a custom humanoid.