freezeing/locking a subform according to a table entry . . . .

I have a form with system details in it. Within that i have the subform Components. Once the system is sold a check box is checked, (in the table its changed to yes under the systemsold thing) I want the subform to become frozen so it can no longer be changed, or at least while the sold box of the overall system is checked.

I want it only to be frozen for that system entrie tho, and when the next system record is viewed for it to be frozen or unfrozen according to that entries Sold tick box.

So far i have had no luck and would really apreciate some help!!! Altho Im not totaly shure i explaned that well: i am very new to access and doing this as a school project. Id love it if someone could tell me :
a) if this is possible
b) how to? If its not something so horribly complicated ill neva understand it!!!

THANX!!!