conspiracist 
noun
A person who supports a conspiracy theory: conspiracists sniffed a double murder, possibly by government agents...
- But that hasn't dissuaded a loose-knit coterie of online conspiracists, antiwar activists and Democratic Party operatives from keeping the draft rumor alive.
- The better known, though still mysterious, Catholic organisation Opus Dei, long a staple of conspiracists, also features prominently.
- At the end, we were left with the unasked question: which are more wearingly tedious, secret loonies who try to walk through walls or whiny conspiracists who live to discover them?