In December, I corresponded with FreshService support regarding this issue and was informed that, at the moment, there is no option to limit this lookup.

​@Hubert.Panszczyk thanks I thought as much, you can grey it out but...I’ll update if i get a different feedback

​@MarkItamsys 
I found those emails in my inbox :) If nothing has changed since then, unfortunately, we don’t have many options to manage this

Best you can do is to enable this and they can search for existing requesters. 

​@Daniel Söderlund thanks however i assume that in this case you can’t key in a requested for Ill test

Unfortunately worse in some ways as you can key in any email, to limit issues it seems as though the safest is to limit the email domains allowed (which will avoid external emails). Support have linked my email to an existing feature enhancement