If you visit the following page and click on the Sync every year link, it should effectively disable Auto-sync. It sets the MaximumTimeBetweenSync parameter to 1 year. I added an action to my Inbox on my iPad after running this about 5 hours ago, and it did not appear in my Mac Inbox until I just forced a sync on the iPad. If you leave the program running for 1 year, it would do a sync on its own, but I don't think that is likely to happen!
I don't plan to keep the link alive forever, so here's the source code:
<title>DEBUG: No Auto-Sync</title>
<meta name="viewport" content="width=320" />
<body style="width: 300">
<p style="font-size: 140%"><a name="reset-keychain" href="x-omnifocus-debug:reset-default:MaximumTimeBetweenSync">Reset sync to default interval</a></p>
<p style="font-size: 140%"><a name="reset-keychain" href="x-omnifocus-debug:set-default:MaximumTimeBetweenSync:31536000">Sync every year</a></p>