Safe mode isn't enabled unless you boot with the SHIFT key held down until the spinning gear appears on the startup screen. There is no other way to boot into safe mode.
Why I'm thinking to disable Safe mode is security.
Just like in Single User Mode, user can get root privilege, and there's a soft way to "block" it to let only administrator login. Is there any security issue when user start in Safe Mode? If yes, there should be a way to disbale it.
There's no different security issues booted into Safe Mode than booted into a normal account. Single-user mode is quite different because then the user is 'root.' However, if all other users than yourself have separate standard accounts, then they cannot make alterations that would require admin or 'root' access unless they know the admin's password. Giving other users admin access or knowledge of the admin's password would be a security problem, and a security mistake.
If you have no other users on the machine but simply wish to prevent a casual walker by from using the computer when it's unattended, then require a password to wake the computer from sleep and/or only operate out of a standard account rather than the admin account.
If you are concerned someone could restart the computer to gain access, then do what Kiraly suggests and use a Firmware Password.
