We describe and evaluate a method to mitigate unwanted student collaborations in assessments, which we recently implemented in a second-year undergraduate mathematics module. The method requires a list of specific pairs of students to be prevented from collaborating, which we constructed based on the results of previous assessments. We converted the list into an undirected graph -- in which vertices represent students and adjacent vertices represent pairs of students to be prevented from collaborating -- and determined a proper colouring of this graph. Subsequently, we designed multi-version assessments, the number and the distribution of the versions being as prescribed by the proper colouring. To assess this method's effectiveness, we conducted an anonymous survey collecting the students' opinions. The results showed that the method indeed reduced collaborations, and that it did so more significantly in the case of quizzes than take-home assignments.