Question

How is it possible to clear the field if too many questionable characters, that way the verifier can just key fresh without the need to clear the field manually?

Answer

This approach can be implemented via a custom script. The pseudo-code will look as follows:
 

// check if the percentage of suspicious characters is higher than some margin (e.g. 70%)
foreach (character in the field)
{
if (character.IsSuspicious==true)
{
SuspiciousCharactersCounter=SuspiciousCharactersCounter+1;
}
}  
SuspiciousCharactersPercentage = SuspiciousCharactersCounter/TotalCharactersCounter*100;
if (SuspiciousCharactersPercentage >70)
{
this.Field("VerifiedFieldName").Value = "";
}