I had this same issue when using Woocommerce with event tickets plus. This is happening because the event tickets plugin is creating the quantity field with a min value of 0 instead of 1. IMO they should fix this on the plugin itself, but you can override the min value with the following code in your theme’s functions.php:
function evcal_fix_min_qty_args( $args, $product ) {
if ( $args['min_value'] == 0 ) {
$args['min_value'] = 1;
}
return $args;
}
add_filter( 'woocommerce_quantity_input_args', 'evcal_fix_min_qty_args', 10, 2 );