Yes there is a way to suppress the K-1 letter

If you want to remove the K-1 transmittal letter from all Addtl K1 Packages, select Settings\Options, then choose the tab "Items to Print." On the left side panel under "Copy," choose "Addtl K1 Package" and simply unclick the box for "Sch K-1 Transmittal Letter."

If your question is just for a single client, go to the General tab, select tab 3.1 Invoice, Letter, Filing Instructions. On the left panel select Beneficiary Letter and at the top, enter -1 to suppress the K-1 letter. (This is from the Fiduciary module, but there should be similar options for S corporations or Partnerships.)