I’ll Just Keep Pushing Until You Say Yes

Have you ever encountered someone that absolutely HAS to have everything their way?  I’m dealing with one of those right now.  It is a family friend, fun, kind, and generous to a fault.  The problem is if they ask for a favor or help with something and it’s something you can’t do they will push and push and push until you give in.  I’m the first one to jump in to help if people need it, within the bounds of reason.  The favor this friend needs is a time sensitive daily committment that we’re just not capable of living up to but they are relentless about trying to convince us.  I’ve tried explaining the many reasons we’re unable to do this but nothing sticks.  I sincerely hope this persistence doesn’t ruin a friendship but we’re running out of options.  How do you deal with someone who won’t take no for an answer?