You can script something to implement what you are trying to achieve and call this script whenever you need to.
Try the following steps:
1. Setup your EC2 command line API tools
2. To use API tools, set up EC2_CERT and also the EC2_PRIVATE_KEY
3. Parse the result of ec2-describe-snapshots
4. Delete the snapshots of your choice using the following command:
ec2-delete-snapshot snap-xxxxxxxx