Like nailing Jell-O to a cross American British informal
Extremely tough or unattainable, to the extent that attempting could be seen as stupid or useless.
Try to debate with such a stubborn person like her is like nailing Jell-O to a cross.
Explaining the theory of evolution to kids is like nailing Jell-O to a cross.
You have violated the rules and of course, you will be docked wages. All your attempts during the half-month are like nailing Jell-O to a cross.
Once something has been done, you can do nothing but face the consequences.
I know you regret cheating on your test but you can't unring a bell.