// convert to a data object, using a lossy conversion to ASCII
NSData *asciiEncoded = [yourOriginalString dataUsingEncoding:NSASCIIStringEncoding
                         allowLossyConversion:YES];

// take the data object and recreate a string using the lossy conversion
NSString *other = [[NSString alloc] initWithData:asciiEncoded
                                        encoding:NSASCIIStringEncoding];
// relinquish ownership
[other autorelease];

Via Stackoverflow.com