Welcome to the Zoom Community, @frodojrr.
I'd encourage you to gather as much information as you can and submit a report to the Zoom Trust & Safety team at https://zoom.us/trust-form.
See this Zoom Support article for additional information, especially the "How to report participants for a past meeting or webinar" section:
https://support.zoom.com/hc/en/article?id=zm_kb&sysparm_article=KB0067474
As you may have already discovered, there is a way for a Host/Co-Host to delete individual chat messages if the option is enabled for your account, but there is currently no way to remove all chat messages from an individual.
To enable the delete chat feature, see this Zoom Support article:
https://support.zoom.com/hc/en/article?id=zm_kb&sysparm_article=KB0058421