Learn Icelandic: Ekki pissa á flísarnar í búningsklefanum! - Do not pee on the tiles in the changing room!

Because this is a negative command. In Icelandic, you form a prohibition by using ekki followed by the bare infinitive of the verb. You only use the imperative form like pissaðu for positive commands.

Why is flísarnar in the accusative case here instead of the dative?

The preposition á takes the accusative case when an action involves movement or direction onto a surface. Because the action is directed onto the tiles, it triggers the accusative. If something were just sitting statically on the tiles, it would take the dative.

Since the action involves movement onto the tiles, why is búningsklefanum in the dative case after í?

While the action is directed onto the tiles, the whole event is happening statically inside the changing room. There is no movement into the changing room itself, so í takes the dative case to show a fixed location.

How is the compound word búningsklefanum put together?

It is made up of búningur (suit or uniform) and klefi (cabin or small room). The -s connects them, and -num is the attached definite article for a dative masculine singular noun.