Hi Emily, this function ( to exclude certain transactions from your targets ) is currently being tested in “Monzo Labs” prior to roll out to every user once it has been tweeked by user feedback - anybody can sign up for Monzo Labs and if you don’t like it its as easy to reverse to original targets - if you want to give it a go 
In the Monzo Labs spending screen , tap on the transaction, scroll down the screen at it will give you a toggle option to exclude this transaction