Gwen gets more and more "Yandere-ish" the longer you go without paying special attention to her.
You can do what Bolt said if you want all the Yandere stuff gone, or - if you want to keep the Yandere system on - you just have to spend a few days with Gwen (doing naughty things, or dating her in the club) to get her level to drop back down to normal levels. Once she's "normal" you just have to be naughty or club-date her once a week to keep her there.
Or you can just keep ignoring her permanently. She has three phases, and so long as you stay in phase two or three she'll only text you ten times before stopping. Also, you only get the night time scene once each time you enter phase three.
Finally, you can just talk to her in her room and select "About you texting me..." and then "Please stop texting me."