In find mode you can use the status area to select whether the find request will find or omit records. It's a simple little check-box.
I have a solution where the status area is hidden. I have a list view of addresses. I've written the following script to replicate that litle check box.
When they enter Find Mode I take them through to a duplicate list view layout where at the end of every row/record there is an extra field called TABLE::Omit. It's a checkbox field with the value 1.
When they hit return my script runs...
// This loop tells FileMaker to 'omit' the request in it's find routine. It leaves all others as 'include'
Go to Record/Request/Page [First]
If [TABLE::Omit = 1]
Go to Record/Request/Page [Next; Exit after last]
// This then clears the Omit field before the find routine
Set Field [TABLE::Omit; ""]
Replace Field Contents
Perform Find 
Any suggestions of turning this into a global script so that I don't have to roll out multiple versions changing the reference to TABLE::Omit/