While there is undoubtedly shared responsibility when dealing with the outside world, multiples have the luxury inside their own head and with some people outside their body to have individuals in their system take individual responsibility for their activites and choices.
Individual responsibility is usually outlined in the House Rules for the system, whether or not such rules are written/spoken or simply understood and accepted as being common courtesy within a system. Violations of house rules can vary from minor through serious, and individuals within the system are generally held accountable within the system for such violations.
The ways of dealing with infractions will vary, but one example is that problem persons' thoughts or actions may be monitored, and Front may be forcibly taken away if they violate the house rules. Temporarily stripping someone of their ability to Front is usually sufficient, and similar to giving the person jail time or a time out.
There have been court cases in which the defense has attempted to foist individual responsibility onto the legal repercussions, claiming for example that the one person in the system who committed the wrong ought to be the only one blamed or punished for a crime. With much deliberation and some reluctance in convincing cases, it was ruled that individuals within multiple systems do not enjoy complete individual autonomy from one another. All the more incentive to have strong house rules and to keep to a principle of shared responsibility in the majority of dealings outside of one's body. It is important to discipline headmates if they violate one's house rules, and to apply individual responsibility to avoid people misbehaving.