how to delete an array of values?

Thanks for this module, it's a life saver! Simple question but I can't figure it out... is there a way to empty the entire array of values?

My node has a user reference field with unlimited values, and I want to remove all of those values (ie, not clone that field). I suppose I can go count the number of values and empty them all, but it shouldn't be that difficult. I've tried the following lines, but none work. Is it a syntax error, or do I really have to count the values and delete each one?

$node->field_reviewers = NULL;
$node->field_reviewers[] = NULL;
unset($node->field_reviewers);