In the literal sense there is no way to "enable" or "disable" async, as the independent engines are basically a core concept of DX12 and it is also allowed to freely mix graphics and compute within a single command queue. Any DX12 compatible GPU has to have a way to deal with that (otherwise it would not be compatible). But MS didn't specify how a GPU or its driver has to deal with it. A complete serialization into a single command stream is perfectly fine, as long as the explicit synchronization done by the developer is obeyed.According to NVIDIA, async is already disabled on Maxwell.
Hence, the only thing nV (or AMD for that matter) could do, is to change the way their driver (or the GPU itself) handles this. But there is no feature flag for "Async" which you could switch off with a driver update and it can't be used anymore.