They never sent an Admiral to go an get rid of Luffy. Green Bull did that on his own accord. Infact plotwise, it suggests that they didn't want to get rid of Luffy, because they dubbed him a Yonko. The WG is all about maintaining the balance, which is why Kaido and Big Mom were replaced immediately following their downfall.
Not to mention, they have an interest in Wano, which has yet to be explored. They aren't going to nuke an island off the map, when that Island has something they want.