Wow, lots of replies.
I know 'no candles' would be a rather daft clause (and, as I said, it's not a clause we have - it was just the LL's comment).
What I'm wondering is whether LLs really have an issue here, and the response seems to be that very few would. For what it's worth, I think our candles aren't very smoky, and I've repainted walls in previous places and noticed smoke damage (in that case, from a very dodgy heater, so not our fault!).
I agree with others saying to pearl that it's not practical to expect tenants to avoid LL who put unenforceable clauses in the agreement. I've pointed out to agents that certain clauses aren't enforceable in the past, but it's never fun and you only do it if you can risk not getting that property/being asked to leave when time's up.
I don't think it would ever work.
What might work, would be educating people about their rights as tenants, rather than trying to get people not to sign contracts. When I was 22 I rented a room in a shared house where the LL's mother (elderly, and, we later realised, in the early stages of dementia) insisted on coming round every few days, letting herself in, moving things in the kitchen, and throwing away anything she deemed unnecessary. Her 'rules' included no shampoo bottles on the edges of the bath tub, and nothing left out on the counter tops. She also assigned us each our own shelf in the kitchen cupboard.
Predictably, I went apeshit, but all the other tenants in the house were shocked and told me she was entitled to do whatever she liked in her own house. They really didn't believe they had any rights at all.