Yes, but in Visio 2003 it requires changes to the Windows Registry. Please see the following Knowledge Base article:
In Visio 2007 it's much easier and you can use this technique for any COM Add-in program for Visio. Select Tools>Trust Center then Add-ins. Towards the bottom of the screen select COM Add-ins and then click Go. You will see a list of the installed COM Add-ins (like the VisiTools Toolbar) and you can check which ones you want active or disabled. You can change this setting at any time to bring the Toolbar back.