I was going to comment that it should be fairly easy to get them off of the rig, but after doing some quick research, it's not that easy.
According to this answer about boarding a vessel without the captain;s permission, "boarding a ship without the permission of the captain or an officer or crewmember designated by him is a violation of international maritime law". It goes on to say that such boarders can be considered stowaways at minimum or even be charged for piracy in some countries, as this same group was last year in the Pechora Sea.
The point that makes it difficult, though, is that if the boarding occurs "...for the purposes of saving a life, preventing a personal injury or property damage...", then it is not considered piracy. I can already see claims that they were preventing damage to the Artic region.
The other point is that it would also make for bad publicity if the boarders were treated badly.
Statoil said in a statement it wouldn't drill into oil reserves of the targeted Hoop area in the Barents Sea until a complaint from Greenpeace has been assessed by the Norwegian Ministry of Climate and Environment
Greenpeace boards rig heading for Barents Sea:
'via Blog this'