Just curious.

Kill innocents.

Additionally, you can pick up Viconia the drow to lose 2 reputation points.

Picking up Viconia drops your rep by 2. I think if you turn over Valygar to the cowled wizards your rep will also drop by 2.

Yeah, that one too.
And there are some other such quests, but I can't recall any at the moment.

Killing Mazzy when she turns against you gives rep -1

The Druid grove quest get's - rep as well if you do the evil option

Claim the ransom for te noblewoman in the people-burried-alive-quest. Ask an advance from the mayor of Imnesvale... They're all good ways of losing or not gaining rep.

Under the copper coranet there is a homeless man with a carrion crawler. Slay him and your rep drops.