Blocking someone on pixiv will prevent them from doing the following:
- Supporting your plans, starting from the following month
- Commenting on your posts
- Messaging you (messages you exchanged up to now will be deleted)
From the month after you block a certain user, they will only be able to view your All users posts.
On desktop, blocking can be done via your User settings (https://www.pixiv.net/setting_access_block.php).
On mobile, blocking can be done from that specific user's profile page.
Check this page for details on blocking users on pixiv.