It's 'useless' trying to change her mind. = It's 'useless' to try to change her mind.
Drugs are 'useless' in treating the condition.
I made a 'useless' attempt at fixing the leak.
(..) Who is it that the monster's taken, Minerva? Ginny Weasley. ''Her skeleton will lie in the Chamber forever. '' Ginny. Lockhart may be useless, but he's going to try and get into the Chamber. At least we can tell him what we know. Professor, we have some information for you. Are you going somewhere? (..)2002 Harry Potter and the Chamber of Secrets