You need an array of enforcement tools. If you involve the right people with enough power and influence, then I think you can find ways to effectively deal with any of the problems being mentioned. For example, you can hire infiltrators to go after delinquent account holders, set bounties, use judges, bankers, rulers, priests, etc. In extreme cases, you might resort to military force. Lords that try to exact unfair taxes may get their guild house closed down, or they may wake to find an assassin in their bed chamber, or they might have their lordship taken away by the duke. Perhaps guild members could be asked/told not to sell food or provide any assistance to lords that have been blacklisted by the bank. The key is to have a multitude of tools at your disposal and get the right people on your side.
