Hi,
Maxwell renders the image reducing the noise with each sampling level.
The longer it renders = more sampling levels = less noise will appear on the image. So buy a faster computer or be patient
In general to reduce noise (from Maxwell's FAQ) :
Make sure your emitting surfaces are as low-poly as possible. Usually, a single-sided polygon will be sufficient.
If possible, do not completely enclose your emitters inside a dielectric object.
Make sure your emitter is not intersecting other geometry.
Very important: do not include very white or fully saturated materials in your scene. For example, a completely white (255, 255, 255) material will take a very long time to clear and will also make the contrast in the image disappear. Also avoid completely saturated colors such as pure red (255, 0, 0). Pure white or pure saturated materials do not exist in real life so it's better to decrease the saturation a bit. For a white wall, around RGB 220 or less is sufficient.
For interior renders where window glass reflections are necessary, use the Architectural Glass Solution (AGS) for the windows instead of real glass. This will create the reflections but will not produce any caustics, thus speeding up the rendering. - Perhaps you simply need more time for rendering, depending on your machine specs.