I try not to be petty, I really do. But a former boss who constantly told me I would never amount to anything, made regular homophobic jokes, and was just generally a shithead to me and my coworkers reached out to ask if I'd put a word in for him at my current employer
I probably should have just stopped at giving an honest and professional summary to the hiring manager. But in this one case, I definitely decided to be petty and I don't regret it for even a second
I pulled zero punches. His candidacy has been pulled, and the hiring manager told him that the reason was: based on my assessment of his character, she had serious concerns about his professional judgement
I probably shouldn't be happy. But I am.
@yojimbo yeah I feel like being honest was the right thing. Being… really, really honest was probably crossing over into "petty" territory 
@calcifer this is Karma. I am working in a small city where almost everyone with decent connections to other professionals knows someone from each other company, so "you always meet twice" - and you really do.
And as others said, you have to protect your team from people with toxic behaviour, so you did the right thing.
@calcifer sure sounds like you did the right thing. be happy!
@calcifer Excellent.
@calcifer This is important - this was an application to your current employer and you have a duty of care to your co-workers to not expose them to a harmful influence.
Our company's hiring process tries to get as many people in the interview process as possible, and operates on a blackball - if anyone says "no", it's "no" for the whole company.
You would probably feel more conflicted if you were issuing such feedback to a third-party, but as it stands, it sounds like you did the right thing