Can a person who once supported pseudoscience or conspiracy theories ever be considered a credible source again?
Let me answer this by turning it around: just because somebody hasn't supported obvious bunk doesn't mean they are even vaguely right.

One of the beauties of science is that, at the end of the day, the people don't matter to the validity of the material.  As long as a document presents a clear argument, you can evaluate it for yourself, no matter whether it was presented by a known crackpot or  a heroically revered elder.  

That means that somebody can be ridiculously wrong in one area---even to the degree of crazy conspiracy theories---and still well respected in another, through the magic of mental compartmentalization and cognitive dissonance.

I had a direct experience of this as a young graduate student: one of the early conferences that I attended was a complex systems meeting with a very loose policy for acceptance: you basically had to be sane to give a talk, but even crazies could join the poster session. I was all charged up from earlier listening to keynotes from seriously famous luminaries, when I walked into the poster session and met a respected mathematician with a poster explaining the connection between fractals and peace, supported by poems pleading for one to use the hypotenuse and not the legs of the triangle for the sake of peace. Complete gibberish.  This person, however, is apparently a well-respected researcher in their own field---they just go off and let their crazy flag fly in certain other places.

That said, if you know that somebody has gone to crazy-town, you certainly would be well advised to examine their arguments and citations quite carefully.  Still, at the end of the day, the only truly unrecoverable stain is falsification, because then people can no longer evaluate your arguments without wondering whether the data is simply fictional.