How do I remove the head off this Giacomini thermostatic radiator valve?
In the technical guides for the Giacomini valves, it appears that some of the heads can be removed by unscrewing it past the the open position. Have you tried that? This is assuming that the top part on your model spins.

For other models, it appears the whole head pulls off the valve. An example is shown in this guide: http://www.giacomini.com/export/sites/default/en/download_gallery/certifications/cen_en_215_tech_doc/1257763387429_R452_1-502GB.pdf

In your photo, I can see the tops of the numbers on the valve, but not enough to compare to the various units the manufacturer produces. You might call them for more info at their support line, here: 

TECHNICAL MANAGER
eng. SERVILIO GIORIA
tel.: 0322 923 372
servilio.gioria@giacomini.com

TECHNICAL DEPARTMENT SECRETARY

Mrs. PAOLA FRATTINI
tel.0322 923 372 - fax 0322 923 255
paola.frattini@giacomini.com