Calling someone a conspiracy theorist is the psychological equivalent of a four letter word. Therefore dealing swiftly with this common ad-hominem attack is essential. Here is a 3 step approach:

1/ Define 'Conspiracy Theorist'?
2/ Why does it apply to me?
3/ Of all people, why does it apply to me and me only?

Here's an example:

Denialist: You're a conspiracy theorist!

Truther: Please define what you mean by conspiracy theorist?

Denialist: A conspiracy theorist is someone who believes that his government killed its own population as a pretext for war.

Truther: How does that apply to me?

Denialist: You believe that our government plotted to kill Americans on 9/11.

Truther: So you're saying that anybody who believes that who believes that his government killed its own population as a pretext for war, is a conspiracy theorist? In that case, all historians having studied Operation Himmler's Reichstag, Nero, Gleiwitz incident, Lavon affair, KGB Chechnya bombings, Algerian civil war, Stalin are all conspiracy theorists?

Just an example, but you can apply this template to any definition the Denialist gives you.