Hello Mário,
you could configure an offset-list under the RIP process to increase the metric of the routs you do not want to use.
Let´s say you have three RIP routes:
192.168.1.0
192.168.2.0
192.168.3.0
and you want to prefer 192.168.1.0, you would configure an access list matching the routes you do not want:
access-list 1 permit 192.168.2.0
access-list 1 permit 192.168.3.0
and apply the following command under the RIP process:
offset-list 1 out 10
this would increase the metric of the matched routes by 10.
Or you can apply the offset to routes learned from a specific interface:
offset-list 1 in 10 Serial 0
Check the following link for the complete syntax of this command:
offset-list (RIP)
http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_command_reference_chapter09186a00801a80c3.html#wp1058911
HTH,
Georg