Perhaps it is a company policy or someone on your IT team feels it is important to block IPv6 tunneling across your network to 6to4 relays or Teredo relay servers or perhaps you only want internal folks using your delegated IPv6 address block. Do you have any options available to you to block this sort of traffic?
As it seems with all things tech related the answer is, it depends on what you want to do. Both 6to4 and ISATAP utilize IPv4 protocol 41 to tunnel their traffic. Therefore, it is easy enough to block IPv6 protocol 41 from traversing internally (which would stop ISATAP and 6to4) or at the edge firewall (which would stop 6to4 but might not stop ISATAP.) On Cisco IOS it might look like this on an internal router or switch:
access-list 100 deny 41 any any
access-list 100 permit any any (or whatever traffic you DO want to permit)
In addition, to this step you can blackhole the IPv4 route to 22.214.171.124 which is the IPv4 anycast address used for the 6to4 IPv4 relay. On Cisco IOS you could do:
ip route 126.96.36.199 255.255.255.255 null0
Teredo clients can be blocked in a simple method because by design it utilizes UDP over IPv4 to establish and build it's NAT traversal tunnel traffic. Simply blocking outbound UDP traffic solves the problem but certainly breaks a lot of other functions for end client machines.
If you are running a Microsoft Windows AD configuration with clients belonging to the domain you can poison the Teredo entry that is used by default on a Microsoft client machine. All Microsoft clients from Windows XP on up make use of the dns name teredo.ipv6.microsoft.com to resolve if they can utilize Teredo to build out an IPv6 connection. This likely isn't the best method but it can be effective and some might say required because from Windows Vista on up Teredo is enabled by default but is inactive. This means if an application gets installed that wants to make use of Teredo it activates the Teredo client and attempts to use it.
You can also use a GPO to change the registry keys to keep Teredo off. You can push firewall changes to the Windows clients (Vista and Windows 7) that would block Teredo or you could turn off IPv6 which would solve the problem also. Microsoft has documentation on all of those options, you can start looking here, here or here to find out more.
So, in those cases where you actually need to turn off IPv6 tunneling technologies there are options available. The next question is do you really want to block these technologies?