Net ionic equation for a synthesis reaction?
You are right, you can't write ionic equation for that reaction. Quick search showed me that even the mechanism doesn't really involve any kind of ions, which, I'd say, is completely logical, considering reaction conditions, which favor homolytic, not heterolytic  bond cleavage.

You can only write ionic equations for reactions where ions are in charge, and they are preferably formed in polar medium.

P. S. The mechanism of S+O2 reaction isn't researched much, so here is the study of H2S+O2 reaction .