There are certain genre tropes that have to be respected in Urban Fantasy and these are the game mechanics to enforce it.
Oaths
When an oath is given by any supernatural being, it is something more than simply a few words. Oaths are “enforced” among the supernatural and when violated they can cause grave consequences. Violation leads to a ban. The effects of a ban vary but usually involve a penalty to all skill rolls (-2 Penalty Skill Levels) that do not relent until the violation is corrected. They may also result in diminishing powers. It is an important consideration that how the oath is obtained is immaterial and that only the letter of the oath must be fulfilled.
Thresholds
Some places are invested with a spiritual energy that protects the inhabitants - homes, churches, and memorials. The Twilight, from which most supernatural powers come, actively defends these sanctified places by lessening the powers of those who do not belong there and in some cases outright forbidding entry. The two most common examples are “The Invitation” and “Consecration.”
The Invitation: Supernatural Beings who enter a home (a true home, not a house) will find that their powers are considerably diminished (25-75% damage reduction). Supernatural Creatures (demons, vampires, spirits) may not be able to force their way through a strong threshold.
Consecration: Holy ground prevents entry to some Supernatural Creatures and legend holds that sites with strong faith may act when that faith is violated [for example: a Buddhist temple may cause violent spells to backfire on the caster].
Memorials: Sites dedicated to honoring the dead exert a subtle calming effect (bonuses to avoid enraged or berserk). These must be active sites where people visit [for example, the Vietnam Wall, The Holocaust Museum]
Bindings
Supernatural Creatures are subject to different laws because they draw their power from the Twilight. One of the primary examples of this is bindings and arcane circles. Mortals are totally unaffected by a powered circle but for the supernatural they are walls that simply cannot be broken down. A supernatural placed into a warding circle may not affect anything on the outside of that circle nor may supernatural creatures affect anything within a circle. It takes 2 phases to ad lib a warding circle.
Neutral Grounds
In general, supernatural creatures respect neutral ground. Supernatural creatures, even those bent on total annihilation of others, must have some venue where they can meet to discuss important items without the constant threat of violence. Vampires call it the Law of Ahimsa, wizards have their Accordlands - the important point is that nearly every party understands the necessity of such places and act vigorously to punish those who violate the peace. Punishment for violations is usually death and even enemies will work together to see that it is swiftly carried out.
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Laws of the Arcanum
Wizards shall not consort nor summon that which lies beyond.
Wizards shall not tamper with the flow of time.
Wizards shall not draw attention to our kind.
Respect the Laws of Hospitality.
Respect the Privileges Accorded to the Tribunes and Sentinels.
All wizards in good standing shall attend the Sabbat and any Hunts.
The first two rules are meant to prevent disaster of catastrophic magnitudes. Altering time can create an unraveling paradox which many believe could destroy the time stream. Any contact with being from beyond our reality [Elder Gods, Old Ones, Kings of Edom] threaten the very resistance of mankind - nothing is worth drawing their attention to the mortal realms. The penalty for violation of the first two laws is a swift death.
The Third Law is far more mushy. No one wants another witch hunt - the remnants of the last one still claim the lives of wizards today through the
Malleus Maleficarum. Empowering them by drawing supporters to their side is detrimental to everyone.
Law four through six mean that wizards should act like they are part of something when the need arises. Hospitality requires that you all rest and respite for any wizard who formally requests it for a duration of no less than 48 hours (in general, this applies to anyone not just wizards). It is considered a breach of Hospitality to request sanctuary and not reveal that you are bringing trouble. Accorded Privileges means that you honor the positions of the Arcanum. Sabbat are meeting of the local wizardry to discuss arcane issues and to take roll of all local wizards, it is permissible to send a letter explaining your absence. A Hunt is called when a wizard has violated the Laws of the Arcanum or Neutral Ground. Every wizard shall use his means to find the guilty party and see them dead or captured. This mostly means dead because no one wants kid gloves on when tackling another wizard.
Rules of the Pack