Going to court is a lot of effort (at least in mine country). The best solution would be to keep the website on your servers, and hand it over after the payment

Going to court is a lot of effort (at least in mine country). The best solution would be to keep the website on your servers, and hand it over after the payment, just like it happens in case of e.g. grocery store - you don't get anything until you pay for it. If anything else fails then trashing the website is quite a good solution - all in all it's your website, until they pay for it.