Unless it`s a good planet that you wouldn`t want to plunder ANYWAY... in which case, one would advise you against plundering it. ^^;
You can also get rid of one by clustering it with other planets.
As well... not all x.class planets are plunderable by virals. A viral can ONLY plunder a x.class planet if it is infectable - however, if it is an x.u.class planet, only marauders and collectives can plunder it.