Wow. I briefly played with searching for these, but was skeptical if there was such a thing. This is one of the greatest discoveries in Life ever. (Though perhaps of little "practical" importance in building other things?)
Well perhaps - like many other seemingly impractical discoveries - the methods used and invented to achieve the breakthrough will themselves find application elsewhere.
It's easy to stop one of these knightships -- an eater has been built already -- and just as easy to make a knightship detector. However, a gun to make new "Sir Robin"s is not likely to appear any time soon: there are construction problems that are thousands of times easier, that are not anywhere near being solved yet.