wp-includes/simplepie/cache

Hi

Which subfolders specifically have filled up? Regardless of the folder though realize that if you have actions occurring on your domain that you did not authorize it’s usually a strong indicator that something is wrong. By wrong I mean, it’s a strong indicator that you’re dealing with a hack. Usually when folders get filled with junk it’s technique used with spam campaigns.

You might want to consider reading through this guide: https://sucuri.net/guides/how-to-clean-hacked-wordpress to help you think through the delousing process.

Thanks

Tony