"portability" of IP Addresses

  Can a provider forbid their customer from announcing allocated
  networks to another provider?

Our contract prohibits customers from advertising space allocated
PA by us to anyone else (or causing it to be advertised by anyone else).
We did this as we concluded it wasn't practically possible to do this
in any non-contractual manner. Also from them advertising anyone elses
address space.

Alex Bligh
Xara Networks