You need to turn CAPTCHA on for an anonymous form in the Dynamics Portal.
You are receiving a LOT of SPAM because your Portal form is open to everyone, and someone is continually hitting it with a bot and creating a LOT of cases.
Turn CAPTCHA on to alleviate the bot traffic.
If you are using an Entity Form
Go to Portals | Entity Forms and pick the form that you want to modify. Under the Form Options section, click on “Turn On Captcha”:
If you are using a Web Form
Go to Portals | Web Forms and click the form you want to modify. Then, under Steps, pick the web form related to the first step.
On that Web Form Step page, under Form Options, click on “Add Captcha”: