Where is this phone registered? Depending on that you'll have a different method.
It's just about taking out the Transfer softkey from the softkey template on the phone when it's in a connected, Ring in mode. This is on CME. You need to use ephone-template.
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucme/admin/configuration/guide/cmeadm/cmetemps.html#pgfId-1010683
You'll find the same kind of settings under Device -> Device Settings -> Softkey Template on the CUCM.
Here you'll have to copy one of the existing templates and modify them as per your wish. Then assign this newly created softkey template to the phones where you want to disable transfer. This can be done using BAT if you want make changes on all 7906 only.
Here's an admin guide for 7.x. You can find one for the respective CUCM version you are running.
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/admin/7_1_2/ccmcfg/bccm-712-cm/b06skey.html
