I don't see why he would just be able to strip her power away like nothing, or imprisoned her on a whim.
Just because he stripped Thor of his power doesn't mean he can do it to someone more powerful than Thor. If he could then why would he say things like
"Her violent appetite grew beyond my control"
"I couldn't stop her, so I imprisoned her. I locked her away"
I feel like if he could just strip her powers like he did to thor, then it would have been pretty easy for him to control her violent appetite and/or stop her, which he admitted he couldn't do.
Also we have no idea how he imprisoned her, what he used or what help he had. We do know that he sent the entire Valkyrie army after Hela first rather than facing her himself.
As for him keeping her imprisoned, I've always thought that he kept her locked away thru some sort of spell or enchantment that would only be removed/broken upon his death. Not that he was constantly powering what ever means he used to lock her away (if that was the case why wouldn't she have been freed when he was in the Odin sleep, or under lokis spell?) or because she was afraid to come back.
"My life was all that held her back, but my time has come. I can not keep her away any longer."
I always interpreted that as what ever spell or enchntment he used was tied to his life and when he dies she will be free, and his time was up.
Im not arguing that she would beat him, just that he just can't strip her powers or imprison her whenever he feels like it.