Use the context menu. There is an option to "hide" (delete) the ad there.

Another option is this bookmarklet (only works if the element that should be deleted isn't in an iframe):

    javascript:(function(){var e=document.body.style.cursor;document.body.style.cursor="crosshair";var t=document.createElement("div");var n="border:1px solid #3280FF;background-color:rgba(50,128,255,0.5);position:absolute;z-index:2147483647;display:none;";var r="pointer-events:none;";var i="transition:width 60ms,height 60ms,left 60ms,top 60ms;";n+=r+"-webkit-"+r+"-moz-"+r;n+=i+"-webkit-"+i+"-moz-"+i;t.setAttribute("style",n);document.body.appendChild(t);var s=null;var o=function(e){var n=e.target;if(n!==s&&n.parentNode){var r=n.getBoundingClientRect();var i=document.documentElement;var o=document.body;var u=i.clientTop||o.clientTop||0;var a=i.clientLeft||o.clientLeft||0;var f=window.pageYOffset||i.scrollTop||o.scrollTop;var l=window.pageXOffset||i.scrollLeft||o.scrollLeft;var c=l-a+r.left-1;var h=f-u+r.top-1;t.style.display="block";t.style.left=c+"px";t.style.top=h+"px";t.style.width=r.width+"px";t.style.height=r.height+"px";s=n}};var u=function(n){document.body.style.cursor=e;if(n.target.parentNode)n.target.parentNode.removeChild(n.target);if(t.parentNode)t.parentNode.removeChild(t);window.removeEventListener("click",u,false);window.removeEventListener("mouseover",o,false);n.stopPropagation();n.preventDefault()};window.addEventListener("mouseover",o,false);window.addEventListener("click",u,false)})();void(0)

Personally, I just don't have Flash enabled at all – it's a security win and I haven't heard my laptop's fans while browsing in ages.