There are several actions you can take on your own if you are encountering an annoying user.
Clicking on their avatar name will bring up a menu that will allow you to mute them. You can also take yourself to a different area if they persist in annoying you. If they are breaking the Terms of Service, you should also report them.
If you are the owner of record for the space you are in, you have other options. In the island management menu, you can edit the island's information and put the user on the ban list. This will prevent them from entering the space. You can also make the space completely private and only accesible by guest list.