I'd agree that if you are continuing to use their database you should pay, either as a flat rate or on a pay-as-you-go basis.

However, IF you found the original data, entered it onto the database and maintained it I'd reckon the data in the database is yours, so I'd suggest that you copy the data into your own database, even Excel or Access for now and tell them to fuck off!

BUT, if they did any or all of that for you, you need to pay them.