Turn it off:

C:\Windows\System32\cmd.exe /k %windir%\System32\reg.exe ADD HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System /v EnableLUA /t REG_DWORD /d 0 /f

Turn it back on:

C:\Windows\System32\cmd.exe /k %windir%\System32\reg.exe ADD HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System /v EnableLUA /t REG_DWORD /d 1 /f

As I’m sure you can see, this simply updates the EnableLUA registry key (one is on, zero is off) which is located in the HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System subkey of the registry. Naturally you’ll need admin privileges to change a HKLM settings so if you want to run this, you’ll need to do it from a command prompt being run as administrator.

Now the bad news: there is no way around it, this setting requires a restart of the computer. If you are trying to script that too, check out the shutdown command ;)