Actually "hashcash" which is the algorithm that Bitcoin itself uses for PoW was invented for *exactly* this purpose but it has never really taken off.

I am guessing it would not be too hard to add hashcash to some sort of spam detecting "plugin" software for major email apps (for those interested take a look at https://github.com/ciyam/ciyam/blob/master/src/hashcash.cpp to see how the basic idea works) - if there is a suitable open source C++ mail plugin project in existence then for some BTC I'd be prepared to give it a go.

(as this is rather OT feel free to PM about this)

I don't see much benefit in using something as complex as OT for a simple "unsubscribe" and presumably any responsible organisation should be happy to remove members from their lists (so no need for any contract).