pixiv is a service that allows you to communicate through contents. You may receive messages from users whom you have not interacted with before and who are not on your My pixiv, don't follow you or are not followed by you.
If you receive a message from someone you haven't corresponded with over contents of which the content is rude or doesn't follow common etiquette, please report the user through the Report Spam Message to pixiv Office button on the bottom right of the message. After confirming the content of the report, a response will be provided if required.
Please check Article 13 of the user agreement for the actions prohibited on pixiv.
If you do not wish to receive messages from a particular user, please use the Block function.
More details about Blocking other users
If you prefer not to receive messages at all, you can limit the accounts you receive messages from via "Message settings".