Hi,
There is no standard functionality to accomplish this.
As a workaround you have to do few configurations
1. Since it is a credit memo, insert a manual price with accepted amount based on the cancellation date for example 50% or 20%
2. To create partial Credit memo's from the Credit memo Request , modify the copy control at the item category with Positive and negative field as Blank
3. Create a User status profile with CLSD ( for example) and assign to the Credit memo request. In the user status profile against your status maintain business transaction As Forbid Invoice creation . By doing this system will not allow you to create Credit memo if the user status set to CLSD. You can set the CLSD user status once you confirm that you don't want to create Credit Memo's anymore
The above process has to be handled manually and cannot be controlled automatically.
thanks
santosh