How are you starting your hangs? Do you need to jump from something or can you just step forward from a chair or ladder? This could be one reason. It's harder to not swing when you have to start your hang with a sudden movement.
Next, examine your form. Hanging isn't simply hanging, it's also engaging your core and pulling your shoulders into their sockets to stabilise yourself. If your legs swing a lot, you can try to mimic a hollow hold while hanging, aka, brace your core, tense your legs and bring your feet slightly forwards.
Imo, swinging legs isn't so bad for deadhangs but could make flex hangs harder. If you feel that this could be a problem, do negatives instead of flex hangs. Personally, I think they help more building towards a proper pull-up than flex hangs.

lofivelcro My pull-up is in my garage, which is kinda short in height, so for all the exercises, I have to raise my legs a bit to not touch the ground. So basically for the hangs, I just grab the bar and raise my legs. But if I understand correctly, during the hangs I have to pull myself up? Just enough for the shoulders to be tight? But generally, I think you are right. I should jump straight to negatives.

Australian pullups are a good training towards real pullups. You go through the movement almost correct and you can adjust the difficulty by changing the heigth (seat of the chair, back of the chair,...) of the bar (broom, stick,...). I consider hangs rather a workout for your forearms than your lats.For a beginner they also might lead to bad form, shoulders not engaged enought ... Like lofivelcro said, negatives (from a small ladder or step, to have a more controlled movemant than a jump up) or maybe assisted, with a resistance band, are the way to go.