How to disable the payment system? I need HBook just for booking enquiries.

You might wish not to take online payment. It can be for example that you manage payment by bank transfer or that you wish to use the system for booking enquiries.

You can select that "Customer do not need to pay online to book an accommodation" in  HBook->Payments. 

Note that you can change any text displayed by HBook in HBook->Text, so you can adjust the text to clearly show your customer that it is a booking request they are sending.