You can stop your Ad spend at any time.  To do this, simply pause your Ad Campaigns.   To pause your Ad Campaigns, go to the "Manage Campaigns" tab, and click the "Actions" button under each Campaign you have running and click the "Pause Campaign" link.

Please note, pausing your Ad Campaigns will not stop your monthly subscription fee from being billed.  As long as you have PPC Optimizer, you will be billed monthly for your subscription fee, whether your Campaigns are paused or not.