You need to guide a government reform to fix the disfunctional state of roman governance that allowed Caesar to rise.
You will change nothing if enough people still believe a dictatorship would be favourable over their current government.

On a different note, given that christianity was born from the times of disruption following the change in governance in rome, you might also stop a religion like it from forming if you keep rome stable.

… why do you say that?

You can’t stop conqueror from conquering.

Other than the assassination of course