We have some spam questions with spam answer...is it sufficient to only flag the questions or we should also flag the answer in order that the moderators can take a look?
Flag both the question, and the answers for that question, if they are spam. That would help in noticing there are other spammer accounts that need to be destroyed.
When I see a flag for a post being spam, I go to the user account page for the account used to write the post, and I destroy the account from there. Having a flag for each spammer makes destroying the accounts faster. I don't even need to mark the mark the flags are useful, as that is automatically done when the account is destroyed.