I just used your site and it made the URL longer. http://zipurl.co.uk/3x/
It would be easier to just take the site down then go through that trouble, unless it's somehow generating a bunch of revenue to make it worth it. Otherwise, if it doesn't jeopardize your hosting account, I probably wouldn't worry about it too much.
If it's clear what the site is for, I don't see how you'd be responsible for the links. I'd think that if one of your links was reported to DH, they'd figure out what was really going on.
Just leaving the redirecting page up should tip most people off that the link they clicked isn't where they're being sent.
In the end, the site the link leads to is what should be dealt with, and/or the person emailing the link, since he's the spammer. So, one of those would be the user's host's responsibility, while the other would be the ISP's. If they do their jobs, your short link becomes useless on its own.
Maybe require an account to make links? That way, it shows a human took time to make the account, then confirmed it through a valid email address.
Then, if they use it for spam links, you might be able to kill 20 spam links by simply disabling one account.
It's a hassle for a spammer that needs to keep making links, but a quick easy process for those that actually have legit uses for it and can keep adding links without breaking the TOS.
If you didn't want to require an account, you could have restrictions for those that don't use them. Examples would be that the link expires in a short time, limit to 1 (or whatever) links per IP per day. Or anything else that would annoy spammers, but encourage legit users to create an account.
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