You misunderstand what an "unknown talkgroup" is. An "unknown talkgroup" is one that is not programmed into the radio, hence it by definition cannot have a service type.
You can have talkgroups programmed into your radio but always avoided (locked out) or their service types disabled, and they will not show up in ID Search mode. I use my radio like this all the time.
I have a system that has talkgroups with the following service types in it:
Fire Dispatch
Fire Tac
Fire Talk
EMS Dispatch
EMS Tac
EMS Talk
Law Dispatch
Law Tac
Law Talk
Interop
Emergency Ops
Corrections
Utilities
Public Works
Transportation
Of those service types, I currently have Transportation, Public Works, Utilities, and Corrections turned off/disabled. I have the Law ones enabled, but in this particular system, I have all the law talkgroups either locked out/avoided or shut off using the quick key functions, because I don't want to listen to this system's law agency. The system is in ID Search mode and always has been. I only hear fire, EMS, and interop/emergency ops on this system, and the *rare* unknown/undiscovered talkgroup. I do not hear any of the other ones programmed in.
SUMMARY:
If you want to be able to pick up talkgroups that are not programmed into your scanner, you need to have the radio on ID Search. Otherwise, you should put it on ID Scan.
If you have the radio in ID Search and you don't want to hear some talkgroups that you're hitting on, you need to save those talkgroups in the system and either avoid them or turn off the service type that you assign to those talkgroups when you save them.
There is no way, and nor should there be a way, to assign service types to talkgroups which are not programmed into the scanner.