Right now, it’s possible to do a server change and then go in WvW, even on servers you are actually facing.
This is used by some to gain unfair advantage over the opponent (telling false information in chat, seizing a siege weapon so it cannot be used, etc.). There are some posts about that already on the WvW subsection. But even if not actively doing acts of sabotage, the simple fact that an opponent is on our map is already disrupting the match. Just being able to see the “team” and “map” chat gives knowledge on where the opponent is, and this intel can be instantly given to guildmates either by guild chat (which Anet can see and so control), and ventrilo/mumble/ts (which Anet has no control over).
When the server changes will stop to be free, we can expect this to happen less often, but with gold to gems transfer, it will still be doable by most guilds.
To prevent this, you should tag a player on a server at the beginning of a matchup. For the entire duration of that matchup, he can only go to WvW for that server. If he decides to change server, he then cannot go to WvW until the matchup change.
That is, in my opinion, the only way to stop those acts of sabotage that are completely ruining the fun of everyone in WvW
