Some options are:
- Saying, sorry, we don't really know what the requirement is, so come back when you have more to say.
- Spend the next couple of hours discussing how to implement all of the options you think of, and planning how to get them done in detail
- List some options for what the customer might really mean, then delegate someone to fine out more, using your options as a basis for conversation.
Option third option is a good compromise. Spend some time discussing what problems the customer might want to solve focusing on the problem, not the solution (implementation). Then spend a few minutes figuring out how you might implement each proposed option so that you can attach a cost to each. Then delegate someone to have a follow up conversation with the customer using your options as a starting point. Three options is a good rule of thumb.
It's very easy to get caught up in solving problems without asking if you're solving the right problem. Whenever you're asked to to build something very specific, ask yourself if you really understand the problem. By taking a step back you can save time, and in the end have happier customers.
(For more on figuring out what the problem really is see the appropriately named book: Are Your Lights On?: How to Figure Out What the Problem Really Is)