I'm using K2 for my article items and when users click the thumbnail it enlarges in a lightbox and I can't find a way to disable this, does anyone know?

Do you use K2 template overrides? If yes, open the item.php in the default folder and locate itemImageBlock block. There you will have to remove the <a> element which is responsible of the lightbox.

Don't remove all contents of the <a> element. Inside it there is the actual image.

That worked a treat cheers! :)