the only way to ensure it, is to make your magic band only have credits on it and not allow it to be used for purchases (don't authorize a credit card on it)..
that way, the only thing they can charge on your magic band is the dining plan credit..
i had exactly the same problem as you, so on the last trip i just didn't authorize using the magic band/key card as a payment system...just dining credits only..
but to answer your question, this is what to do on the receipt..
typically, the receipt will have an amount...and then below it there is a blank line for a tip, and then another blank line for the total..
you should put a line across the tip line - don't leave it blank...actually put a line through it...
and then put the total in the total line...
so say, your bill was $20..
put a line through the tip line..
and then write in $20 into the total line...
this should be ok....
but not necessarily...
the only fullproof method is not authorizing your magic band/key card for charging
.