The device can only bind to one cloud account, do you share the account with your housemates?
You can delete the bulbs from this account which all of you are using, then bind it to your own account. Then you can control your bulbs on your own.

I would guess he's on a shared network, they've installed the app and are activating the switches locally. The only solution I could think of would be for him to set it up a separate network to which only he has access.

No they definitely don't have access to the account as I only just set it up. We are all on one network, is there no way to lock it down to just my account rather than anyone on the same LAN, seems like a massive oversight.Will look into setting up on a separate network but that's going to need changing the router to one that supports VLAN's (and I'm not in control of the router) or setting up a second router and double NATing (not ideal).

Kindly try this: Go to KASA app and enable Remote Control for your devicces, on KASA page, click on the Bulb name and you will see timer, schedule, usage, at the top of the page, click on the three lines icon on the right side to go to the device settings page, then enable Remote Control.
After that, the other clients cannot find your devices on their app even they are in same network.
Can try this and see if it helps:)

I found the option however with this option on it stops my Echo from controlling the as well which is how I control the bulb 99% of the time.

Stop family members from turning on and off the light with remote control technology. So what is it?

means only the one logged into the cloud account can see and control the smart devices.