The Imperial Cruiser is a ship designed to fight in a war and kill enemy combatants. Therefore, it symbolizes violence. The end.
What? Why are you still here? We're done. Imperial Cruiser = violence.
Okay, okay, guess we can discuss this a bit more.
After Anacreon discovers the Imperial Cruiser, Prince Regent Wienis sends it along to the Foundation to be spruced up. Why? Because he plans to use the cruiser to attack and conquer the Foundation, duh.
And Hardin knows this perfectly well, but he orders the cruiser fixed and delivered to Anacreon all the same. Wienis thinks he's won the day. After all, the cruiser contains more killing power than any ship in Foundation's fleet, so war, a.k.a. violence, will ensure him the victory.
Not so fast. The cruiser is run and operated by men trained at the Foundation, men who believe the Foundation to be a holy place. Hardin knows this. He also knows that without men to operate the ship—i.e., soldiers willing to perform violent acts for the sake of their country—then the cruiser is just bolts and metal and circuitry. The ship itself isn't harmful, only the intentions of the operators. When its operators decide they don't want to follow through with the violence, then the ship becomes a harmless piece of nonviolence floating in space.
You know, guns don't kill people: people kill people. (Although it sure is a lot easier with guns—er, an Imperial Cruiser.)
In the end, the cruiser still represents violence, but it also demonstrates Hardin's motto: "Violence is the last refuge of the incompetent" (III.1.55). Wienis tries to use the ship against Hardin as a tool of war because that's all he knows. He doesn't understand religion, science, technology, or even the hearts and minds of his own people. He is, in a word, incompetent.
Hardin also uses the ship against Wienis, but since Hardin is an educated man, he understands all those things that Wienis slept through in school. With this knowledge, Hardin manages to defeat Wienis with the ship without resorting to violence. So, we could say that education and knowledge are the first refuge of the competent.