Wiki says imprisoning them should work, and ironically suggests excommunicating them to accomplish that?
Also, with free investiture, you could nominate the son as successor to "an old or accident-prone bishop"
Currently playing as Pictland (Skotland (Scotland)) from a Charlemagne start, first time I've seriously played in a long time. It's great. Starting as tribal let me mess around with the familiar raiding mechanics, though I wasn't able to raid like an 867 Scandinavian. It has tactical benefits like softening up a county before declaring formal war, though, and the extra gold was nice.
It's now 806 and I've completely vassalized Pictland, which isn't that impressive considering I started as king. I did capture the castle of Dun Ber and kick out the count in order to adopt feudalism, and then Tanistry, so I only suffered one gavelkind succession. The results of that gavelkind (a strong rebellious relative to the north) were actually solved very handily when I adopted feudalism, due to the way it wipes out your levies. Suddenly the pretender's "faction" (just him) had absurd relative power, so he pressed his demands. In actuality my levies were growing fast enough that I probably didn't need to hire a mercenary band, but I did anyway.
Of course, now I'm in the somewhat unfamiliar position of being a Christian king surrounded by Christian realms. I'm forging claims, obviously, and slowly making headway into England that way. Trying to get just enough to usurp duchies then press those de jure claims. It's slower than being a pagan but not so bad.
I'm tantalized by the idea of asking the Pope for claims, but it's not working yet. Probably because I have free investiture to make my nobles happy, which I think adds a strong extra penalty when asking the Pope for favors. On top of the general opinion modifier which I've mostly overcome with my court chaplains. I would try to install one of my bishops, but I don't know how to even get them into the college of cardinals. I don't think it's particularly practical for a fairly poor nation of picts... Though I guess I could create a vassal theocracy under a virtuous bishop.
And I don't think an antipope can offer claims like the real Pope. Though I'd love to see the heresy spread across West Francia, hehe.
I'm just a bit worried about the Norsemen coming up around 867, I guess. I don't think I'll have Britain established in 60 years, but I should be about done with England with luck.