Don’t blame and villainize the poacher. Folks, it takes two to tango.
Someone who poaches your guildies to another guild cannot poach the unwilling. Soliciting your guildies away is kind of a dick move, but the ultimate decision rests with the “poachees.”
We all have free will.
Now, go read Gquits by Cat at Dwarfbabble. This is an excellent analysis of the types of gquits that are encountered in a guild and the most productive way to react to them and move forward.