Hello, I was on a creative server, when the brush somehow got bound to a block (minecraft:blackstone). The server is not mine and they use FAWE. If I try //brush none it says, brushes can't be bound to blocks. I tried reconnecting, it also doesn't work.
You will need to get in touch with the server owner, we can't help you with that issue here since we can't tell you how to fix that since you would need access to server logs and plugin files most likely.