Serial non-participation is a response behavior that is frequently found in stated choice experiments. One form of serial non-participation is that a varying number of respondents chooses always the zero-price or status quo alternative. The approaches used in the literature to deal with this problem vary from excluding those respondents to using applying latent class models to endogenously allow for different preference structures. However, latent class models also allow to assign respondents to a known class. In this paper we compare specifications of latent class models with and without restrictions. Additionally, we control for differences in the error variance across respondents by applying a scale-extended latent class model. The comparisons are designed to show whether respondents are allocated differently among classes and whether willingness to pay estimates are affected significantly. The data are from a choice experiment regarding management actions to enhance forest biodiversity. The data set is characterized by a high percentage of respondents who were not willing to pay; almost 50% of the respondents always chose the zero price alternative locating this sample rather at the top of the range of respondents who are not willing to pay.