Use an auto-enter calculation such as:
Filter ( Self ; "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")
To strip out the undesirable characters. If there are other characters such as punctuation marks or lowercase letters that also need to be paste-able, add them to the text in quotes. Also, clear the "do not replace existing value..." option.
I noticed today I'm having justa tad of a problem with the filter calculation. I did just as you stated but when I enter (Not Paste)AAAABBBD12345 as the serial number the calculation stripped out the leading alpha characters. When I copy & paste this same text string it works just fine.
Look for some other cause for the missing character unless it was a lower case letter that started your text. (The function as I wrote it filters out lowercase letters.)
Auto-enter calcs should work the same for both pasting and typing data into the field.