Was this a particular channel/app? I've never seen a Roku say, "Welcome back" and you don't sign into a Roku account on a Roku device. Each channel/app may have a sign in, but not each device.
I was about to post the same thing. You probably just need to log out of your account on that particular channel. Look for an "Account (often just an icon shaped like a head)", "Options", "Settings", or similar link to click, then look for some way to "Log out", "Disconnect" or something similar.
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well. If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X