The hell??....Players Attacking me and I cant't fight back??

I have the little problem that during grinding in the field it happens sometimes that other players come and attack me, that in itself isn't the problem here, the funny part is that I can't attack them even though they are beating the hell out of me. Is this an actual feature, bug, glich, hack, cheat or something and if it should be legal how can I fight back on those guys?

just hold ctrl.
then click the people that you want to attack..