Sentence : I dislike being ignored.
Would you label this as active or passive? I (sub) dislike (verb, seemingly active) being ignored (verb, passive structure)
Is there a rule that states the "main verb" denotes the sentence is active or passive.

This is an active sentence.

The term "being ignored" is a gerund phrase. In effect, it's a noun not a verb.

So, your sentences is no different in structure to this:

I like cakes.

Subject - verb - object

"I dislike being ignored" is exactly the same.