There is something like "implicit" opt-out.
After making a comment on someone else photo, you start receiving comments from other users about this photo in your comment stream. And after reaching certain number of other users comments (I think it is 10 pages of comments) following your comment, you are opt-outed.
However, you cannot opt-out on your demand (other than deleting your comment, what shall be IMHO used as a last resort only).
The way how the comments are propagated to the comments streams of other involved and interested users is the way how Panoramio tries to make interaction viral (to attract users to more interaction) and promote photos made by other users... when you visit someone's profile, you often see photos of other users in this user comment stream.
Of course, it could be distracting and contra productive engagement for someone.
So if you don't like it, don't comment. But call to leave is over the edge, IMHO.