netstat -tunlp

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 0.0.0.0:389 0.0.0.0:* LISTEN 2226/slapd
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 2215/sshd
tcp6 0 0 :::389 :::* LISTEN 2226/slapd
tcp6 0 0 :::22 :::* LISTEN 2215/sshd

and lsmod showed something like this:

Module Size Used by
ipv6 235364 12
...

So, to disable IPv6 I changed /etc/modprobe.d/aliases:
...
# alias net-pf-10 ipv6
# Disable ipv6
alias net-pf-10 off
alias ipv6 off
...

I also disabled these lines in /etc/hosts to avoid confusions:

...
## The following lines are desirable for IPv6 capable hosts
#::1 localhost ip6-localhost ip6-loopback
#fe00::0 ip6-localnet
#ff00::0 ip6-mcastprefix
#ff02::1 ip6-allnodes
#ff02::2 ip6-allrouters
#ff02::3 ip6-allhosts

Finally I restarted the server.

shutdown -r now

Now the situation is like this:

netstat -tunlp

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 0.0.0.0:389 0.0.0.0:* LISTEN 2233/slapd
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 2216/sshd

lsmod|grep ipv6

...

No IPv6.