Does anyone have a hint to keep my boules from spreading out while in the final rise?
If it's a high hydration dough, a banneton/brotform is most helpful. Actually, it's helpful even without a high hydration dough. You can use a linen napkin, flour it well (with rice flour preferably), then place it in a bowl to create a makeshift banneton to support the boule during the proofing. When the dough is ready to be baked, place a square of parchment over the top, then your peel on top of that, and flip the peel and bowl over. The boule should release easily so you can score, load, and bake (and steam, if you use steam).
Good shaping plays a part as well. Don't know if you checked out the tutorial on the front page titled Shaping a boule. If not, take a look.
I believe that shaping is most important getting the boule to expand vertically as well as horizontally. You must develope an oriented skin on the boule during shaping by stretching while shaping. Pushing and patting does no good. This is important even when you bake in a loaf pan.