I'm gonna answer these in the most simple way possible, others can elaborate:
1. If the unrelated site actually featured you for something - ex. you sponsored a program for this travel company, they mention it on their site and link back to you - then that's fine. If it's totally random and not related, it's not fine. If links to you have anchor text that's not related to your site, that's spam.
2. Almost always disavow at the domain level. Exceptions would be if someone set up a spammy blogspot.com or tumblr.com page - those are big, authoritative domains and you don't want to block them completely. In that case I'd go with the URL level instead of domain level.
3. I'd still disavow the links, even if you delete the pages they point to. It's better to cover all bases in a situation like this.