I came across this page on their website. However, Nook devices do not have access to Android Security settings (without root).
What I would try, and not sure if it will work, but might as well give it a shot:
1) Go to your profile menu > Edit Profile > Manage content > uncheck Sophos under Apps > Save
2) Restart your Nook
3) Because Sophos is unchecked it should not run at startup (this is where I could be wrong)
4) Try to uninstall again
After that I got nuthin'
I'd suggest going into settings and finding the application there. All of my apps display buttons for: "Force Stop" and "Uninstall".
Those may work.