This isn't the first offense. They were fined $65M in Dec 2020 for similar infractions [1]. Clearly that wasn't enough to deter bad behavior. I'm a fan of exponential backoff. Pick an exponent. 3x-10x the last infraction seems reasonable (assuming similar levels of harm to users).
[1] https://www.cbsnews.com/news/robinhood-sec-fine-65-million/