How do I ruin my own base?

The best way to "ruin" it is to start a new account and remove the old one. Then the Town Hall is gone forever.

You can't downgrade buildings / upgrades, but you can make the Hall an easier raid target by separating all the buildings as far apart as possible. Similarly, you can avoid building or upgrade anything so as to not improve your hall. Essentially, do not play the game after making your base as easy to raid as possible.