No, it can't be greyed out, but you can disable change by using the "prevent change" user token policy option.
I guess you could move the box outside the bounds of the window by editing the theme file, but we would not want you to do that as it would make you difficult to support.
Your call, but just don't log a ticket asking where the button went...
I ran into so many issues with this. The main one being the initial post of users not being able to access network items after thinking they changed their network password, when they really changed their pre-boot password.
To get around this, I disabled the ability to change their password at pre-boot, but I also made sure the 'Require Endpoint Encryption Logon" setting. This will give you back the windows logon prompts when your screen locks, etc.. and should resolve your issue.
In Vista and Windows 7, EEPC removes the picture style Windows login and replaces it with this greyish login prompt, which takes over 10 seconds to appear sometimes and also causes the CTRL+ALT+DELETE text to disappear if you leave the prompt up for about 2 minutes without putting in a password. I have worked closely with McAfee Tier 3 and a SDAT was developed that gets rid of the grey prompt and gives you back the familar password and picture prompt users were familiar with. This also alleviated the waits for the greyish prompt to appear, etc...
I am testing the SDAT and it works well so far.