Field relationships

Is there a way to have a delete record command button on a subform only
delete the field(s) that are linked to the parent forms table? For example:
I have a table (misc_donations) with 3 fields: donation_type, donation_amnt,
donor; and another table (donations_types) that has just 1 filed :
donation_type. On the form to enter misc_donations I include a button to
open a subform to enter/edit donations_type. The donation_type on
misc_donation table is selected from a pull down list. I can get it to
update changes to donation_type but if I delete a donation type from the
donation_type subform it deletes all of the records in the misc_donations
which have that particular donation_type, I just want it to clear out or
change the donation_type field.

Thanks, Kris