Вот маленький грязный пример:
NSString *test = @"Olé, señor!";
NSMutableString *asciiCharacters = [NSMutableString string];
for (NSInteger i = 32; i < 127; i++) {
[asciiCharacters appendFormat:@"%c", i];
}
NSCharacterSet *nonAsciiCharacterSet = [[NSCharacterSet characterSetWithCharactersInString:asciiCharacters] invertedSet];
test = [[test componentsSeparatedByCharactersInSet:nonAsciiCharacterSet] componentsJoinedByString:@""];
NSLog(@"%@", test); // Prints @"Ol, seor!"