The fact that at least some of the remaining Elves, especially those of the Noldor, had the power to fend off or destroy any of Sauron's servants individually (if not Sauron himself) seems evident to me. However, we do not indeed see any use of such power by them, the most gifted of the Children still in mortal lands.
Gildor's excuse for himself and his group was basically that the affairs of Middle-earth were no longer their business. I suppose it
does seem a bit of a cop-out, but I think it was essentially true. They were destined to give way to the Aftercomers. They knew it, and the Last Alliance was their last hurrah.
Glorfindel acted only as a guide for the last few miles to Rivendell. To be fair, he was facing all Nine by the time they reached the Ford, and Gandalf himself did not think Glorfindel and Aragorn combined had any chance against them all.
So why did they not take action against the greatest minions of Evil?
For one thing, there don't appear to be many Elves left who were capable of standing toe to toe with the likes of the Witch-king or the Mouth of Sauron. Gandalf said there were few, even in Rivendell, who could ride openly against the Nine, and if they weren't in Rivendell, that pretty much leaves only Celeborn and Galadriel.
After the Last Alliance it appears most of the High Elves were gone, either killed in battle or removed to the West. Those that were left, I think, knew that their time of action was past, and their place then was to give council and refuge such as they could to the enemies of Sauron who
were taking the fight to him.
Also, as a point of feasibilty, how would Glorfindel or Gildor go about these assassinations?
The first step would be to locate their target, a pretty questionable business on its own (unless the Witch-king had a MySpace page with some useful photos and comments left by his buddies Shelob, Gothmog, and the Mouth of Sauron, whom they all call 'Moz'
)
Even if they knew where one of the higher-ups was likely to be, the location would probably have been Mordor or Minas Morgul, and those don't seem like the sorts of places one could just walk into boldly with at most a handful of companions, and go searching about with abandon, even if they
were on Manwë's Buddy List in the Blessed Realm thousands of years before.