You can apply a dedicated flat coat as suggested, or if you like Future, you can add some Tamiya Flat Base to it, which Dependant on the amount added can give you a final finish anywhere between gloss and dead matt flat.
There are other flattening agents on the market, & Tamiya Flat Base is a much maligned product due to its lack of clear instructions, but that said it works well for me. Flat Base is ADDITIVE, not a finish in itself.
Matt Swan has mixing ratios on his very usefully Future page;
http://www.swannysmodels.com/TheCompleteFuture.html