PaintingsILove is set up with the facility for artists to apply a block themselves when they don't want input from a certain member. Most of the time this works well, and with thousands of artists and thousands of comments a day, this gives artists on the site the power and responsibility to delete comments they don't like and to block members they don't like.
You can block someone by locating one of their comments under one of your paintings and clicking the "delete" link. The next page will give you the option to also block them from commenting again on any of your pictures.
When someone is blocked by three different people, that person will automatically no longer be able to post any further comments anywhere on the site, subject to moderator review.
If you have seen the comment on someone else's picture, you can use the "report" link next to the comment to report it to the moderators. Only members who have made at least 200 comments can report items, and only recent comments can be reported.