Do you have authority over demons?

Why was God mad at Balaam when he appeared to obey? Is “once saved, always saved” actually true? Should I get baptized a second time if I was already baptized as a child? What kind of authority do we have over demons? How should we exercise it? How can I find relief from intrusive thoughts about God and blasphemy? I tend to think in images and pictures. I’m wondering if there’s anything wrong with this?