Hi!
As people already mentioned, certain civs are related to certain terrains, so messing with these would help.
Personally, I have found humans to be much more annoying and difficult to deal with as they will actually settle in forest retreats after they killed the elves.
Anyhow, if you want to eliminate the elves entirely, just increase the number of non-mountainous caves (to say, 600). Odds are, that a giant-infested cave will be near any elven settlement, which means that settlement will be destroyed within less than 40 years. Don't worry, all other races are much more resistant to cave creatures (although they do occasionally lose settlements to them).
Another thing you could do is using the parameters to make grassland and hills near the forests so that humans meet the elves early on. This usually spells the end of the elves.
Currently, I am experimenting to find a configuration with long-living megabeasts and all major races still around at at least the year 600 without any modding. My best was about year 366 or some such with a wonderful set of neighboring human and elven civs constantly fighting each other.
In general, humans and elves very often will fight to the death once they encounter each other - so you usually find human forest retreats near any towns or the ruins of towns near any elven forest retreats. Similarly, dwarves and goblins impact each other negatively, however, I have seen less agressiveness there than with elves and humans (but then again, they tend to have fewer settlements).
If you want hints about terrain in the world gen, require a ridiculously high number of squares of the terrain you want info about. When the game offers you abort, it tells you exactly what features need to combine to create the terrain that is lacking.
Deathworks