Right now, i'm a hunter and on 10man Lich King, when i get sucked into the sword, I have no intterupt, Scatter shot doesn't worki, wyvern sting doesn't work and Freezing trap doesn't work either, I however can kill it just before Terenas dies, but I imagine that it have more health on 25man, and when I get there, I don't know what to do.
Some hunters on the EJ forums have reported that you can Distracting Shot to make the Spirit Warden target you and then Feign Death to drop aggro. In doing so, you can interrupt the Soul Rip channel. I personally can't confirm as I haven't been targeted by Harvest Soul in the past few weeks, but quite a few have mentioned it working (as well as some unable to get it to work) so try it at you own risk.
Our hunter literally just kills his, but I am 90% positive you could DS/FD it as well, because the first time I ever went in there on my warrior I actually just taunted and tanked the spirit and he soul ripped me.