The default penalty for the violation of the own promise (that means, the penalty which is applied if no other penalty is defined for a given promise) is a record about the fact of the promise breaking in the black list, a list which is accessible to everyone.
Each record in the black list contains the following informations:
The whole record is signed by the arbiter. With electronic signatures, it can be verified by the reader automatically that the record is legal, that the rulebreaker has really given the arbiter the right to do what he has done.
One advantage of the black list is that one cannot run away from it or refuse to pay this fine: The arbiter has all he needs to put the decision into the black list, namely the signed statement which gives him the right to do this.
An important property of the black list is that there is no central decision about the seriousness of this. Everybody is free to read the black list, or (more reasonable) let some robot read it, and to decide about his future behaviour relative to this person.
This, in particular, solves the problem of unjust contracts: If you violate an unjust contract, the fact about this violation will appear in the black list. But everybody is free to decide for himself that the contract has been unjust, and, therefore, to ignore the violation of the contract.