Bioweapons are actually easy though, and what prevents you from building them is insufficient practical laboratory skills, not that it's somehow intellectually difficult.

The stuff is so easy that if you wrote a paper about some of these bioweapons, the reason you wouldn't be able to publish it isn't safety, but lack of novelty. Basically, many of these things are high school level. The reason people don't ever make them is that hardly any biology nerds are evil.

There's no way to stop them if they wanted to. We're talking about truly high-school level stuff, both the conceptual ideas and how to actually do it. Stuff involving viruses is obviously university level though.