Is it possible to combust a plasma?
If you specifically mean a hot plasma, I think it might be possible but it's a tough request. To get combustion, you'll need to start off with form of bonding present between atoms in the plasma. This requires finding a molecule with atoms so strongly bound that the bond dissociation energy is higher than the molecule's ionization energy. 

Given that the lowest ionization energy known (3.51 eV or about 340 kJ/mol for ditungsten tetra(hpp)) is comparable to relatively sturdy covalent single bonds, there's not much room for negotiation. Ionization energies easily become larger than even strong covalent double bond dissociation energies, so there's a large tendency for any substance to be stripped into its constituent atoms before getting a chance to ionize.

The situation is easier if you allow the usage of "cold" plasmas. Plasma generators are commonly used to scour surfaces completely clean of any organic residues. I don't see why you couldn't just insert oxygen and some organic gas into such a generator, ionize them both with the strong and rapidly varying electric field inside, and cause the organic gas to combust.