You can customize it via a JavaScript, but i think that the current options are OK.
-> Block anonymous users only You usually want to stop *all* vandalism from that ip. You may want to soft it for proxys, but not usually.
->Prevent account creation Blocks without account creation are mostly useless.
The default you ask allows to bypass default blocks, simply by creating an account (remember: it's instantaneus, you don't even need for an email) and vandalizing with username.
"Walter Vermeir"
Thanks to Tim bug 550 is fixt and now it is possible to be more specific who and what you like to block.
To specify this there are 2 tick boxes with these default settings;
[ ] Block anonymous users only
[X] Prevent account creation
I find these are not good default settings. The default setting should be the softest block possible. And that is only a block for anonymous users and account creation allowed.
So that this would be the default setting;
[X] Block anonymous users only
[ ] Prevent account creation
Can this be specified somewhere by a sysop on the wiki?