These neutral density filters are recommended to reduce moonlight. The following models are available:
Up to 10cm aperture the ND0.3 or ND0.6 is recommended, while for larger scope the ND0.6 or ND0.9, and from 20cm the ND0.9. Choosing the proper filter depends on individual needs as well.
For achromatic refractors the moon filter is recommended which not only reduces the amount of light but somewhat correct the color aberration as well, increasing contrast.
Note that two polarising filters can be used as a variable transmission neutral density filter.