An example of a valid universal negative is "No triangles have four sides". This is a valid universal negative because if something does not have three sides, it cannot be a triangle.
The universal negative claim by atheists that God does not exist
See also: Atheism and arrogance
Contrary to the mistaken notion of individuals who are inexperienced in logic/philosophy, there are plenty of cases where universal negatives can be proven. However, atheists' universal negative claim that God does not exist is not a reasonable universal negative claim.