If someone causes me pain but I don’t lament it, feel sorrow about it- what circumstance, if any, warrants acting against the person causing pain? Step 1: Some person causes me pain 

Step 2: let’s say I stay neutral about pain, pain does not cause me sorrow, and I don’t feel even slightest feelings of ill will towards that person — all because I know pain is impermanent and it is not self 

Step 3: so at that point, should I take action against this person, as matter of cause and effect - or not - or what would Buddha do in similar circumstances
Remaining equanimous doesn’t mean that you shouldn’t take action.

It means not reacting automatically from your emotions.

Step 1) someone hurts you.
Step 2) you observe this and ensure you are equanimous and non reactive.
Step 3) take mindful, conscious action. Not out of pity or anger, but out of compassion and reason.

For example if someone assaults you, it’s not that you should be a passive rag doll. You can defend yourself, but it’s important to have clarity and peace of mind before and while you do it.