Again, there is no point blocking the technology when you have no control over people that can potentially be hired to create the same thing.
Well... it depends.
For instance ARM designed the AArch64 ISA, using some patents held by ARM. This is what allows them to require a license of anyone who implements the ARM ISA, even if you use none of their other IP. If you implement it without a license, they'll sue you for patent infringement.
So, if RISC-V has a patent pool, then someone could be effectively banned from using it, or parts thereof, simply by restricting access to the underlying patents, regardless of whether or not a license is normally required to implement RISC-V. Of course, that would only work in jurisdictions which respect those hypothetical patents. Still, if Chinese company made RISC-V CPUs which implement the vector extension, and that extension depends on a US patent, which the US Government has restricted from being licensed to Chinese entities, then they could at least ban importation of those processors
or anything containing them.
Now, what that wouldn't necessarily prevent is them from designing their own extension (or ground-up ISA) which doesn't rely on that patent. However, not supporting the RISC-V ISA (or an extension, thereof) means they wouldn't be compatible with tools, libraries, and apps which use it. Being shut out of an ecosystem is the
really bad part of all this. It means having to create your own ecosystem, which is costly & time-consuming, and then having to get others to adopt it. All of that can put one at an insurmountable market disadvantage. In fact, from what I've heard, RISC-V isn't so great - it derives its main benefits from its large & growing ecosystem!
It is not easy, but with the capital and time, they will eventually catch up and even overtake with the talents. Whatever they try to block, is at best a speed bump, and not going to stop China from progressing.
If you're talking about something like semiconductor manufacturing, the trouble is that existing players are continuing to evolve quite rapidly. So, it's not enough merely to reach where they're at, today. If you don't want to be at a competitive disadvantage, you have to catch up to where they'll be, several years down the road. That's not easy, especially if it means having to reinvent decades' worth of innovation & infrastructure!
Sure, technology is "leaky", and a lot of the IP has already leaked. That doesn't necessarily make technology sanctions a fruitless exercise. They're not a long-term solution, of course. They're best used in a tactical fashion.