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.
(..) Oh, hey! I have an idea. Since Lucy and I have lots of work to do, why don't you girls go and explore the mall? Bye! Here is some money, go buy some useless mall junk. Some headbands and... Are you gonna marry Lucy? Are you out of your gourd? No! She just works with me. Plus, you love her. (..)2013 Despicable Me 2