Nitrates are a vexing problem. Problem is, even with heavy skimming, it's more about preventing new nitrates from forming but doesn't deal with the existing nitrates very well. The live rock, sand, refugium, deal more with that, on their own rates.
Water changes are probably the best method for a rapid reduction.
People talk of dosing vodka, dosing sugar as methods for feeding bacteria which feed upon the nitrate and then are skimmed out - you can try reading up on those methods.
Supposedly Zeovit, Utralith and etc. etc. are intended to keep nitrates at bay.
Denitrators work but take time to produce a noticeable effect.
Good luck - nitrates are big pain and the enemy of SPS.
I know it all too well.