How do I remove the signature line from MICR checks?

From theAMS Payroll Menu click Payer>Payer Edit>Check/MICR. ClickEdit *.chk.
At the bottom of the file you’ll see a row of numbers, yours may be slightly different:
40 7 00 39 50 26
change the last number (26) to zeros:
40 7 0039 5000
Click Quit/Save.
This changes the length of the signature line to zero.