How can I disable the integrated QuickBooks merchant service ?

QuickBooks POS version 10 and below:
In the Company Preferences | Merchant and Gift Services, please remove the Merchant number.

Newer QuickBooks POS versions (i.e 2013 or newer):

Choose File > Setup InterviewClick the Payments tab. Select No and then click Done.

Other QuickBooks versions (Pro, Premiere, Enterprise):
Make sure the checkbox 'Process ... credit card payment' is unchecked all the time when you save a sales receipt, a receive payment or a credit memo..