If they want to socialize, they will find the way, no matter how many stores you close and whatever else you try to do.
The point is, they haven’t recieved or accepted the Chinuch that
it is better to wait for shidduch than to socialize/flirt that may be so called innocent or may not be.
Having shiurim or programs might work, only if they are socializing out of rebellion, then you need to make NCSY type programs, i.e. “Judaism is cool” approach.
My children went to frum Yeshivos not mixed classes but the boys and girls sharing the same huge buildings, separate wings, and went to completely separate high schools.
I spoke to them about socializing, they think that it is wrong, that the time will come when they are in the Parsha of Shidduchim.
So those who got the same basic Chinuch, are rebelling a bit.
The issue is to facilitate the socializing in a heimish way or not but that might seem hypocritcal but it may be worth exploring.