Can I cross through or make checklist fields unavailable with out deleting?

This is a sign up form for our mission trips. Some of the trip dates have already passed, so i want to eliminate the ability to select the field, without loosing all the data and without creating another form.

What should I do?

If I understand you correctly you would like to remove certain option from your check box field. I did some tests on my end modifying the field. As long as the field still exists, no save data will be affected when changing the options. 

This would mean, that you can modify/remove/add option on the check box field. This will only affect the new records and not affect the old once. If you remove the entire field, you will loose the data.

Please let us know if you require additional assistance.