TYCHAY (I am actually not the developer, but I do work for Plaxo and do use the Mac client.)
Actually, you don't need to set manual syncing to avoid having your contacts modified by other Plaxo members.
Simply go to your Plaxo Account Preferences, communication pane and uncheck "Automatically accept updated information in my address book".
While you are there, I also unchecked "Plaxo Alerts", which cuts down on the stuff coming in my inbox considerably. The problem is if you do so, you must remember to visit My Plaxo and allow/disallow updates coming in individually. (Yes, I know this doesn't work so well in Safari, and that is entirely my fault. Now that the Mac client is out in beta, getting the online experience in Safari is a top priority.)