How to remove certain items of custom fields when sending a offline receipt?
If you want to stop all custom fields appearing in receipts, you can edit the Contribution - Receipt (off-line) template directly, in 'Message Templates'. 

Remove the section starting with {if $customGroup} through to the next {/if}, and give that a try.

If it turns out it's using the on-line receipt (I'm always getting confused as to which is used when), it's the same but with {if $customPost}.