If it is a post they made, you can click the report button just below their name in the post. If it is not then you will need to email the Roll20 team (<a href="mailto:team@roll20.net" rel="nofollow">team@roll20.net</a>) and provide a link to their profile with the reason why you are reporting them. One thing, if the reason happened outside of Roll20, such as on a discord, there is not much they can do about it.