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"Big Brother" contestant Spencer Clawson just learned a valuable lesson about child porn ... don't even joke about that crap ... because cops will go after you.

In case you didn't see the clips from this week -- Clawson grabs the microphone attached to McRae (another houseguest) and while pretending to be McRae, Spencer goes off on this really uncomfortable riff about child porn, saying, "I like to beat off to child porn. Did I ever tell y'all about that? I love it. Beating off to child porn is my favorite thing there is."

You'd think he'd stop there, but no ... he kept going, saying, "I love it when they're around three or four years old. My favorite ones are when you can tell they're in a basement."

And the jokes didn't go unnoticed ... especially by Spencer's hometown police in Arkansas.

Chief AJ Gary from the Conway Police Department in Arkansas tells TMZ, "The Conway Police Department was alerted to comments made. Due to the nature of the comments, our department moved quickly to look into the matter. At this time we haven't found that any criminal act was committed."