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Don't the play the demo.

Until Valve does something about it, make a different Steam account just for trying demos.

Never heard of that issue before, and I have played some demos on Steam without problems. It's either with specific games or something has changed since the last time I tried demos.
Sorry I couldn't help.

I think it happens on free weekend games as well. To make it disapear from your owned list first you have to remove the game from your library(obvious), then you have to go to your recent activities (or games played (something like that)) and you have to play as many different games until the demo disapears from that list. I think that might work

the appid of the full game should be different from the demo, no?