Use a function to replace the smart quotes (following is based on code from this plugin), in functions.php
:
function wpse89095_untexturize( $text ) {
$char_codes = array( '‘', '’', '“', '”', '′', '″' );
$replacements = array( "'", "'", '"', '"', "'", '"' );
return str_replace( $char_codes, $replacements, $text );
}
Then filter your option before it's saved to the database, in functions.php
:
function wpse89095_untexturize_option( $newvalue, $oldvalue ){
$newvalue = wpse89095_untexturize( $newvalue );
return $newvalue;
}
add_filter( 'pre_update_option_{OPTION}', 'wpse89095_untexturize_option', 10, 2 );
NB: replace {OPTION}
with the name of the option you'd like to filter.