Giving services for free (even for one single hour) is a bad idea nowadays, because you will have a lot of hackers, of frauders and bad customers that will want just to try their desctructive scripts on a professional hosting server. Some of them do not even have a credit card, or live in a country/region where payments can not be made via internet.
The best way in my point of view is to give them the possibility to sign up for a small amount (for example 0.5 $ for 6 months). By doing this, you have the ability to verify their identity (via PayPal or your payment processor) and are sure that they are not frauders or duplicate accounts.
In addition to this, PayPal has an automatic subscription option. You can ask them, at the moment of the checkout, to authorize you to renew their account automatically