A simple way around this (as you appear to be using CRM2013) is to remove/hide this lookup from your form and add the 'Account' lookup in its place. This will in-turn update the 'potential customer' field (which is what you are showing here).
Rob
MCTS. GAP Consulting Ltd. Microsoft Community Contributor Award 2011 & 2013
Proposed as answer byRob BoyersEditorSaturday, November 1, 2014 9:08 PM