Well, does it definitely have to be the Kingdom of Syrakuse? If so, it precludes a POD before about 400 BCE when it was still a democracy. The way I see it, Syrakuse is a major target for any expansionistic Italian power; Sicily is fertile and Syrakuse is a major trading nexus for the Western mediterranean. Also, there are few other powers who would seek to protect it, as the Carthaginians are certainly not friendly with the Sicilian Greeks.
So, if you want to avoid Syrakuse becoming incorporated directly into an Empire, one of two things seem likely to cause it; no power successfully unites the Italian peninsula until Syrakuse is powerful enough to resist successfully (or perhaps Syrakuse is involved in the unification of Italy), or a power that can compete directly with an Italy-based Empire that would be friendly to Syrakuse exists.