Hey, Unai, "Kaixo"! and welcome to Tikiwiki Community!
This reminds me of a security hole that was discovered in an old version of tikiwiki, and patched/secured many months ago.
Since Tiki 2.0 is the new official and stable release (after more than 3 years of development), I would suggest you to upgrade your tiki installation straight away, to ensure that you have the most secure and update Tiki stable release.
BTW, looking at the source code of your homepage, it looks like the spammers add that through some modified file in your webserver, not database (I'm not specialist, it's just a guess). If you don't find the hacked file, you could just start over again with your new Tiki 2.0 and upgrade your Tiki 1.9.x database (see docs: Upgrade)
Ask again if in doubt.
Cheers (and "Agur")