Need to have validation for " Assigned to " field in SAP Service cloud.

Hi All,

I have business requirement in which the value of "Assigned to " should not be equal system /technical users . In my service cloud tenant , the value for ticket assigned to is taken from fiD which first changes the ticket and later on this values need to be changed manually to user. As the current rules available are for visible, mandatory , there is no validation to check that user cannot be system user.

Can anyone share any pointers on this requirement.