Behavioural status and demographic characteristics of target groups influence the implementation and effects of interventions to reduce and prevent harm. We examine the implementation of a statutory identification regime and associated monetary limit-setting in the context of electronic gambling machine gambling in Finland. Mandatory identification of players is a prerequisite for various policy measures aimed at preventing and reducing gambling harms. We use a large account-based dataset (N = 28,351) from the state gambling monopoly to examine behavioural differences between those who identified voluntarily before and those who did so only after identification became mandatory. The identification regime was implemented in steps. Consequently, we defined player groups based on different implementation phases. We compare these groups in terms of demographic variables and consumption patterns. Results show that those who identified for the first time only after identification became mandatory experienced the highest average losses. Mandatory loss limits were associated with a clear reduction in overall consumption across all three groups. We conclude that when implementing harm prevention policies, it is important to consider differences across gambler groups. Furthermore, preventive policies need to be rules-based. Voluntary measures, although favoured and actively lobbied by the gambling industry, are not as effective.