Norul asked: Why can't hanafi followers eat seafood?

Answer: Some scholars considered all seafood halal, since it is mentioned in Quran that its harvest is halal [Surah al-Ma'idah 96]

According to Hanafis this is restricted by:
Surah al-'a3raaf 157 which says we cannot eat khabaa'ith, which could perhaps be translated as "vile things". They interpreted this as other than fish (samak). Also, the Prophet (may Allah raise his rank)forbode killing frogs (Ahmad, Bayhaqi, Abu Dawuud, etc.), so its forbidden to eat it. Moreover, he forbode eating what has not been slaughtered except fish and locust (Ahmad, Shaafi3i, ibn Maajah). Thus seafood other than fish is not allowed. Non-Hanafis attacked the above hadiths; it comes back to the methodological questions of when it is allowed to go by a hadith that has some weaknesses.

Note that a "hanafi follower" is a person who has deep and wide knowledge of the school and its evidences. I.e. someone who is convinced that his school is the most likely to be correct. People who are not at that level are simply required to follow a mujtahid, and most of them allowed all seafood, such as shrimp, crab and oyster. The Imam Malik even allowed frogs if they have been slaughtered, but I don't know the details of this.

Norul asked: Can a male wear a gold ring or other type of ring?

As for the ring, a male can wear one silver ring (not gold). This is because the Prophet (may Allah raise his rank) wore one which he used as a seal for documents.