Thanks for the aid mpack, I was uncertain about the reason of this problem and which subforum it belongs. I will have it in mind next time.
According to the discussion of archlinux mentioned earlier, it's controlled with setterm.
I'm not sure why new member could not post links, there should be more effective way to prevent spams for clever programmers.
People may need to register for multiple different forums and ask a lot of questions when they enter a new area. Too much restriction may bring unpleasant and inconvenient feeling to them. BTW, I see the "report spam" button under each post and reply. I wonder whether users here would ignore them.