The "diffraction spikes" you are seeing is the result of using a narrow aperture. Some folks refer to such effects as "sunstars." The light travelling through your lens' aperture diaphragm is interacting with the blades of the aperture. The number of spikes you see is determined by the number of aperture blades. Lenses with even numbers of blades have the same number of spikes as the number of blades. Lenses with odd numbers of blades have twice as many spikes as the number of blades. (In actuality, even numbers of blades also have twice as many spikes but half of them are lined up directly over the other half.) It looks like you are using a lens with seven aperture blades.

To make the lights look rounder you need to open the aperture by selecting a lower f-number. You can also make the lights look a little rounder by slightly defocusing. To decrease the influence of the ambient lights you also need to shorten the length of your exposure(s) and/or reduce your ISO setting by more than enough to offset the wider aperture.