You just need to fill in the form at https://www.tvlicensing.co.uk/no-licence-needed/ every three years. I've done that before and it's stopped the letters.

It didn't work for me, they still sent threatening letters every few weeks that all but accused me of lying.

I love the BBC (even though I didn't always own a TV) but in my experience the TV licencing company are bullying scum.