Regularly deadheading the spent flowers encourages new ones to flower and stops the plant setting seed. The black spot may have come in with them, but it would be difficult to tell. It's more likely to be down to this moist mild weather we are having. You could spray them for black spot.

This happened to my Violas very badly last Autumn. I sprayed them but with no real effect. This year I went back to the bigger Pansy as I find they dont seem to be affected.

My violas in the garden often do that, I just give them a deadheading and trim and the new growth usually comes back ok.

now I did just the opposite Drc!, mine are fine at the moment but I think its a fungus as they just rot off if it attacks them when young