Hi Sephaerius,
I haven't done any InfoPath-Access work yet but I would suggest querying Access with the Account Number first. If the data connection returns data (number of rows > 0), then you could halt the submit process and display an error message like "The Account Number already exists. Please enter a different Account Number". Otherwise (if number of rows = 0), continue with the submit logic. I hope this makes sense.
Regards,