The answer is Group Policy, but as with all security it's a fine line between usability and security. For the Ask toolbar you can simply disable add-ons in IE (provided it's v6 or above) by doing the following (from KB883256):
- Click Start, click Run, type
gpedit.msc, and then click OK.
- Go to Computer Configuration (or User Configuration) → Administrative Templates → Windows Components → Internet Explorer → Security Features, and then click Add-on Management.
As for Chrome, you have a bazillion options in Group Policy to prevent installs, or run-times, or simply change users to non-admins. Sadly not too easy on this one.
Took me a bit to figure out how my users (small base) kept getting the Ask toolbar and Chrome (I don't mind the latter as long as it doesn't add toolbars, change search options, or homepage in IE).