Giacomo1871 says:

To delete all items, make a bookmarklet with following code as an address:

javascript:(function(){ var v = new RegExp(“PersonalDocuments”); if (!v.test(document.URL)) { return false; } {a=document.getElementsByClassName(‘rowBodyCollapsed’);for(var i = 0; i<a.length; i++){Fion.deleteItem(‘deleteItem_’+a[i].getAttribute(‘asin’));};return; }})();

via Amazon.com: Customer Discussions: Request for feature: Ability to delete multiple personal documents in one operation.