。
Don't do anything unnecessary.
Literal
Unnecessary [attributive-な] things [topic-は] do-[negative-imperative-な].
余計なこと ('unnecessary things / things that shouldn't be done') is a sharp idiomatic phrase for meddling or overstepping. The terminal ~な is the abrupt negative imperative attached to a dictionary form — 'don't do.' A terse warning, often from superior to subordinate, in the same register family as 余計なお節介 ('mind your own business').