how do you detune a TV?

Silly question I'm sure but how do you detune a TV? Do you need to
totally make it impossible to get broadcast channels )ie remove the
tuning mechanism) or do you just need to manually tune to TV into no
programmes - I took the aerial out and ran the automatic tuning
programme hoping that I wouln't then get anything but the old staions
popped up bright as before.
What should I do?