If the moon is in the frame, some flare and overexposure are hard to avoid because it’s vastly brighter than the rest of the night scene. You can reduce flare, but not completely eliminate it.

What helps in-camera:

• Use a lens hood to block off-axis stray light.
• Remove unnecessary filters, especially lower-quality or non-multicoated ones, since they often add flare.
• Try stopping down the lens, which can help with some lens aberrations.
• Recompose if possible so the moon is less problematic, or move it out of frame if you don’t need it visible.
• A strong graduated ND filter may reduce the intensity somewhat, but it usually won’t solve the problem entirely.

Important limitation: A “normal-looking” moon and a properly exposed cityscape usually require two different exposures because the brightness difference is so extreme. The common solution is to shoot one exposure for the city and another for the moon, then blend them later. Without that, the moon will usually blow out and may flare.