To give another deviant a direct link to this FAQ answer, use the following code in your comment or forum post:
Can I block people from my profile page?
Each user can block up to a maximum of 500 people for any reason from their userpage.
To do this simply visit the 'Personal > General' section of your Edit Settings page, once there towards the bottom of the page you will see a box entitled "Blocked Users".
Enter the username of each person you wish to block into the field provided, with each username placed on its own separate line. Be certain to correctly spell each name and do not include the status symbol of the user (ie: *,~,+).
Once Blocked, the user which you have listed will be unable to comment on any of your personal areas including your main page, journals, deviations, personal shoutbox (if subscriber) and personal forums (if subscriber) and will be unable to send any Notes to you. In addition the system will prevent a Blocked user from Favoriting any of your submissions, will prevent attempts to deviantWATCH you and remove you from their deviantWATCH list if you have already been added.
Users which you Block will not receive any advance notification that you have done so, however the block will be immediately evident when they visit any of the prohibited commenting areas.
If you feel that the situation is truly serious, please notify the Help Desk, so a staff member can review the situation and take whatever action is deemed necessary. Please note that if you unblock someone to continue communicating with them, you are unable to re-add them to your block list for 48 hours.
PLEASE NOTE: Using alternative accounts to contact people who have blocked you on another account is considered harassment and can result in all your deviantART accounts being suspended or permanently banned.