If you want them to self seed around the garden - leave the flowerhead on the plants. If they are coloured the resulting seedlings will not be as dark as the parents. If you want to cut them off - remove the flower stalks right at the base. The foliage on Primula Denticulata can get rather large during the summer but will die back completely in winter. I find that growing them behind something that will hide them in summer works just fine. I use Lady's Mantle - still dormant when Primulas are flowering and tall enough to hide the scruffy, often slug devoured foliage. Choose a plant that grows suitable in the spot you grow them.