Who does what
To clear up a few things.
For a dual flight to be counted toward the JAA requirement for revalidation then the instructor must sign and also state that the flight satisfied the requirement for revalidation. A signature on its own is not sufficient. The signature otherwise could be for any purpose such as a routine club check etc. At a later date the examiner assesseing relevant experience will not know what the signature represents without the additional short statement.
Further the revalidation flight is for the purpose of ongoing training, it is not a test. The instructor only anotates the log book to confirm the purpose of the flight has been satisified. Only an authorised examiner may examine.
In a free world the instructor cannot be forced to sign if they do not wish. But it is their choice. However not to sign is rather pointless because the valid pilot remains valid just the same and may still have 11 months and 30 days of flying before his class rating need revalidateing by an examiner. If the pilot has refused to co-operate with the training then indeed do not sign but that would be right because the purpose had not been satisfied.
A further bit of detail correction to a DFC comment. The licence is not revalidated. The class rating held is revaildated/renewed. Licence renewal is a seperate issue.
There is no recommedation that the instructor should refuse to sign if they are unhappy. The instructor is not required to assess ability of the pilot for licencing puposes but only to carry out the training as required. However it is left for the Instructor and the qualified pilot to cover the hour as they best believe is most useful. The CAA at one point did devise a recommended content which has since been withdrawn.