The web doesn’t have a native mechanism for protecting content from copyright infringement. Aside from complex digital rights management solutions, which aren’t really appropriate for photography sites, there’s little a photographer can do to prevent determined infringers from copying images and using them without regard for the photographer’s chosen licence.
This is a huge problem for photographers. The web is an essential tool for displaying and promoting photography, but anything published on the web is up for grabs — at least from a technical perspective. From a legal and ethical perspective, photographers have the right and the ability to protect their work. But many users either don’t understand or don’t respect those rights.
There are various routes a photographer can take to the reduce the impact of copyright infringement, but none is ideal and each has trade-offs.