Firstly, regular old smart-contracts, which are self-enforcing, inasmuch as the contract itself takes the form of an AI which acts to enforce/fulfil itself – which can, relevantly, cover anything from simple supervisory work, to suing you for breach and demanding specific performance, to hiring mercenaries or assassins to enforce the contract, if you know what I mean?
The other would be the geas. In the modern, non-mythological sense, that would be a dynamic mind-editing thought-virus that – in the legal consumer versions, that is, that come with an intact obligator seal on them – make the idea of violating the deal you just agreed to of your own volition literally unthinkable1.
Geases are used extremely rarely, and even suggesting that one might be used (in a society where one’s word really is and ought to be enough for anybody) is a very strong insult; but sometimes you do need to be able to make a deal with the untrustworthy, and that’s where these come in.
(A related concept is that of the Cilmínár professional – doctor, advocate, etc., named after the world on which the process was developed – who have used geas technology on themselves to be able to guarantee to their rich and paranoid clients that they are genuinely incapable of betraying their clients’ interests.)
1. One important thing to note here is that you don’t see that stereotypical scene in which someone is doing something while desperately struggling against the geas or other compulsion in order not to. To struggle against doing something, you have to be able to conceptualize the notion of not doing it. With geas technology, you can’t do that.