Why not just check the options before calling dimox_breadcrumbs()
like so:
<?php
// Check if the user wants breadcrumbs, default to "Yes" if no option set
if(strcmp(get_option("to_breadcrumbs", "Yes"), "Yes") == 0){
if (function_exists('dimox_breadcrumbs')) {
dimox_breadcrumbs();
}
}
?>