I wish to know is there in Chinese language a single word for "無用"?
if by 无用 you mean useless and by one word you mean one character then I would recommend:
废 it has a lot of meanings but specifically here it contains the meaning:
waste; useless; disused; superfluous in ²fèizhǐ -ABC
It depends why that thing is considered "無用".
For example, if it is considered redundant, you can say "贅".