Can Lord Shiva kill anyone who is immortal ? Or can Lord Shiva break any boons given by other Devatas ?
Can Lord Shiva kill anyone who is immortal ?
Yes, he can when their time comes as ge is MahaKala himself.
Shiva Purana 7.1.13.:
2-4. The Odd-eyed Śiva is the foremost among the gods. The trident-bearing Śiva is brilliant. Lord Rudra the Kālātman is Kapardin and Nīlalohita. When the end of the yugas arrives he becomes infuriated and annihilates this universe including Brahmā, Viṣṇu and the fire-god. He makes the worlds shrink.
Or can Lord Shiva break any boons given by other Devatas ?
Yes, he can he just don't do that.
Shiva Purana 2.5.40.:
Then in the battle, the powerful lord Śiva suddenly seized his trident which could not be withstood even by brilliant persons, in order to slay him.
In order to prevent him then, an unembodied celestial voice said—“O Śiva, do not hurl the trident now. Please listen to this request.
O Śiva, by all means, you are competent to destroy the entire universe in a trice. What doubt then in regard to a single Dānava Śankhacūḍa?
Still, the limit imposed by the Vedas should not be disregarded by you, the lord. O great god, listen to that. Make it truthful and fruitful.
12-13. O lord Śiva, it has been mentioned by Brahmā, that, as long as he wears the armour of Viṣṇu and as long as his wife maintains the marital fidelity, Śankhacūḍa has neither death nor old age. Please make those words truthful.’
- On hearing this celestial voice, Śiva said “So be it”.
I hope this clarifies all your queries. Prd..