Hi Andre - no it's not possible to reset scores. I see you've achieved your gold shields (well done!) if you aim for Diamond next you'll need to get perfect scores in all of B2 content for this and KwizBot will take you through it all again. 

We are also looking into various mechanisms for re-covering levels that have been perfected, including a voluntary data 'ageing' mechanism and perhaps "Round 2" achievements to give the motivation, whilst keeping your history and data so we can be intelligent about prioritising the practice.