Currently the only way to create an account is through GitHub login (we're going to add more options in the future). That means in order to write a review you have to log in with GitHub.
Behind the scenes, text reviews do not show immediately, we go over them and approve them manually, to verify there's not bad behavior, spam, offensive content, etc.
We plan on approving each text review ourselves manually until we're stretched too thin and this plan doesn't scale anymore.
I don't think anyone has been able to 100% prevent those, so it's still work in progress.
I guess once we cannot do this ourselves, we're going to bring on additional team members to help with this, and once this is not enough, we'll have to use some combination of moderators (not very scalable), crowdsourcing (let users report suspicious reviews), and technology (identify and surface suspicious reviews).
We honestly don't know yet, but we know it's going to be a priority.