We actively seek out people that have received money that aren't users. We're thinking about ways to automate this, but making it personal is probably going to be the most effective.

You can search for a domain, url, or email here to check if it's been tipped: http://tipjoy.com/domainSearch/

If it has, you need to authenticate that you own it through one of a few methods.

Does the tip get debited only after the intended recipient receives it?

No, when the tipper gives it. Also, someone only receives the money into their account when the tip is paid, but they are shown the level of unpaid tips.

I expect a number of blog posts of people saying "hey folks, pay your tips!". We're deploying some friendly ways of encouraging it too.