Hiya --
i am new to this site ,and i am a time served high -end colour copier engineer ,we have customers with this problem day in day out ,it is not normally an issue in day to day copying ,but i work for companies that use machines that cost £40K and use them as a network printer/ scanner for marketing use .
Dont want to teach you to suck eggs ,but unfortunately moire is a very inexact science unless you know your input medium and your scanner intimately .
Firstly
have you tried the descreen button on the manual setting of the twain driver (only works in colour)--this is there for moire --
or Adjust the scan resolution by just a few pixels either way (yes you can type in non standard numbers !!)
or Move the image skew by a few milimeters then scan and correct later
or ,and not too many people know this, try putting a good quality transparency(OHP slide) between the image and the scan bed .
Hope this helps ,but as i said unless you are using the scanner day in day out with the same pic on the same medium,then it is a matter of trial and error
oh and make sure you either phsically or electronically mask any extraneous parts of the image --

Regards
Rich