When I tried to stop the ipsec service, I got this error message:
ipsec_setup ipsec: Device or resource busy
although I did a "ifconfig ipsec0 down" before.
[SOLUTION]
The kernel-module "ipsec" couldn't be unloaded because of encryption modules (e.g.: ipsec_aes, ipsec_sha1, ...) loaded on top of it.
Unload those modules and ipsec can be stopped cleanly.