It can be achieved via data partition.
Follow below steps in hiding impact value '3-Single Group' for a specific role:
1) Go to Administration -> Security and Role Management -> Data partitions -> Data partition List
2) In the right side window select the default data partition for that specific role.
3) In the data partition detail window click on "New Constraint" and give the values as below:
Table Name: Impact
Constraint Type: View
Constraint: sym != '3-Single Group'
Here example taken is for impact value "3-single Group", it needs to substituted by the required sym value accordingly.
Save the changes and login with that role. From the Impact drop down this value will no longer be visible for selection.