Actually, that screenshot you posted isn't a bug... technically.
The way it works is that around 2.5 to 3.0 billion inhabitants (or so) it is impossible to influence a planet, regardless of how many ships you put into orbit. You can still gain influence with raids, however, up to about 3.5 or thereabouts. Past 3.5 your only recourse is to use orbital bombardment to knock them down to size and then work on gaining influence.
However... if you read my "Pirate Tips" thread you'll see a part in there where you can actually build your own Defense Bases over enemy (empire) controlled worlds. Doing this means that you effectively removed that planet from the game without destroying it, because the defense base will vaporize every spaceport and/or construction ship the planet ever tries to build the second after it begins construction. The planet can still grow and produce ground troops, but no ship will ever leave it unless they can find a way to destroy your Defense Base, first.
So what I normally do is go in with a blitz fleet straight for their homeworld (Diplomacy menu is handy that way!), knock out their spaceport and plop a defense base over it. Then I go around and influence all their little starter colonies with ease, and those colonies take a long time to ever build up enough force to make a play at liberating their homeworld (even moreso when I'm leeching half their income away with pirate bases).
All told, you can win a medium sized map with less than 30 warships grouped into just 2 fleets. Considering how nerfed pirate income is, anything you can do to cut costs is always a good thing.