Shifty Shellshock's Baby Mama -- I Want Full Custody ... You Smoked Crack in Our Son's Room!

Shifty Shellshock's baby mama is asking an L.A. judge to take away his visitation rights ... claiming he's a terrible father who once smoked crack in their kid's bedroom.

The "Celebrity Rehab" star's ex Jasmine Lennard filed suit last week against Shifty -- real name Seth Binzer -- seeking full custody ... and removal of his visitation rights.

Lennard says Seth hasn't seen their son since November 2011 -- nor has he paid child support -- so she wants a judge to make her the sole guardian.

Even worse ... Lennard claims Seth is still addicted to a whole slew of drugs ... and his problem is so out of control he once left a crack pipe in their son's room after freebasing.

Jasmine claims she tried to establish Shifty as the son's legal father last year -- but Binzer acted like it wasn't his kid to get out of paying child support, even though he admitted it was his kid on "Celebrity Rehab."

So now, she wants the judge to officially give her full-time custody ... and she wants him to pay up, stat.