Learn Icelandic: Ekki sletta!

To give a negative command in Icelandic, you usually just put ekki in front of the bare infinitive of the verb. You do not need to conjugate it into an imperative form.

Does this phrase change if I am yelling at multiple people to stop splashing?

No, it stays exactly the same! Since the ekki plus infinitive structure does not conjugate for the listener, ekki sletta works whether you are talking to one person or a large group.

What if I wanted to give the opposite command and tell someone to splash?

Positive commands require the actual imperative form of the verb instead of the infinitive. To tell one person to splash, you would say slettu, and to tell a group, you would say slettið.

I have heard the word sletta used when people are talking about languages. What does it mean there?

You will often hear að sletta used to describe mixing foreign words or slang into Icelandic. In that context, ekki sletta is a common way to tell someone to speak pure Icelandic without throwing in English loanwords.