To poison my sandwich knowing that someone else will steal it and eat it.

But if he did it to get revenge, then halachically it is forbidden because it is neither atonement nor a punishment, right?

This is at most revenge in the Grammar. The one who stole the sandwich brought this upon himself.
Beyond that, when a person harms me against Halacha, it is a different matter than someone who does not want to lend me a tool or something like that. See the response of the Maharashtrian 6:5, and also Ramban 19:18. However, the Ramban still qualifies that it is permissible only to repay the damage and not in vain. From the explanation, it seems to me that a thief is not at all the issue, as the Maharashtrian writes.

And putting ink in my shampoo while sitting down, knowing that someone regularly uses my shampoo, is that ethical?

If you have made it clear that it should not be used without permission, there is no problem.