set keyboard type to UIKeyboardTypeASCIICapable (txtField.keyboardType=UIKeyboardTypeASCIICapable;)
you can find swift code from below link

- (BOOL)stringContainsEmoji:(NSString *)string {
    __block BOOL returnValue = NO;
    [string enumerateSubstringsInRange:NSMakeRange(0, [string length]) options:NSStringEnumerationByComposedCharacterSequences usingBlock:
     ^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
         
         const unichar hs = [substring characterAtIndex:0];
         // surrogate pair
         if (0xd800 <= hs && hs  1) {
                 const unichar ls = [substring characterAtIndex:1];
                 const int uc = ((hs - 0xd800) * 0x400) + (ls - 0xdc00) + 0x10000;
                 if (0x1d000 <= uc && uc  1) {
             const unichar ls = [substring characterAtIndex:1];
             if (ls == 0x20e3) {
                 returnValue = YES;
             }
             
         } else {
             // non surrogate
             if (0x2100 <= hs && hs <= 0x27ff) {
                 returnValue = YES;
             } else if (0x2B05 <= hs && hs <= 0x2b07) {
                 returnValue = YES;
             } else if (0x2934 <= hs && hs <= 0x2935) {
                 returnValue = YES;
             } else if (0x3297 <= hs && hs <= 0x3299) {
                 returnValue = YES;
             } else if (hs == 0xa9 || hs == 0xae || hs == 0x303d || hs == 0x3030 || hs == 0x2b55 || hs == 0x2b1c || hs == 0x2b1b || hs == 0x2b50) {
                 returnValue = YES;
             }
         }
     }];
    
    return returnValue;
}

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    if ([appdelegate stringContainsEmoji:string]) {
        return NO;
    }
    if ([textField isFirstResponder])
    {
        if ([[[textField textInputMode] primaryLanguage] isEqualToString:@"emoji"] || ![[textField textInputMode] primaryLanguage])
        {
            return NO;
        }
    }
    return YES;
}