My FIC instructor told me thus:
1.) As an instructor you always have the right to refuse to sign off a two-yearly flight if you deem it to be unsatisfactory but the licence holder has every right to pop down to CAA HQ and get his/her licence revalidated by them direct as they have, in theory, completed the requirements
2.) If you do a flight that aint great and you don't want the responisibility, I was told to tell the individual concerned that you would be signing there logbook but stating that the flight was not to standard. In this way most examiners would be unhappy about signing the licence off and the CAA take the responsibility
Anyway, logbooks look so much more interesting with signatures and stamps all over them!