SemCycle seat twists

I bought a new SemCycle XL a couple of months ago and really like it except for
one thing. I can’t stop the seatpost from twisting. I have really tightened down
the bolt which seems to have even misshapen the seatpost slightly. But the post
still twists. It will turn maybe 5 degrees or so in either direction. This is
really starting to bug me. Any way to get this to stop? I think the chrome
seatpost is just too slippery.

My old Schwinn has a bolt all the way through and that one never turns.

Andy Arhelger