How to disconnect from APP smart glass on mobile without access to the device?

when you change the password, (write it on a paper and save it in a safe place for not forget)

You can change your password so the app is no longer accessible on their phone.

have you tried to change your password for your account so that the signed in account on their phone is no longer valid?

Regards

Too bad friend, see if this link helps you, good luck

https://support.xbox.com/en-US/xbox-one/apps/smartglass-console-setup

You can manage your devices from your Microsoft Account page at account.microsoft.com

From there you can remove what has access to your Microsoft Account and that will revoke access from that device.

This page here has some more details.

Additionally you can simply change the password to your Microsoft Account and that should revoke access from the devices and require anyone to re-sign in with your account to get access again.