Help, team members out of tasks want to start with future sprints topics

I have this problem, a team with 4 members working under scrum, with specific skills each one of them:

Person A) DBA with reporting skills Person B) DBA with stored procedure skills and a very deep knowledge of the business Person C) Developer Person D) Tester

Problem 1

Right now, we're at the end of sprint 6, Person A has ran out of tasks, and he wants to start working on sprint 7 issues. I have explained to the whole team that we must focus on finish sprint 6 functionalities, and not to think about independent tasks, but it is not being really accepted. On despite off they team is not working on sprint 7, they're not really convinced about it.

Problem 2

We will have the sprint 7 planning meeting on wednesday, and the team wants to talk separately to the PO in order to ask him questions about the User Stories, Person C argues that listening to "person A" talk with the PO is a waste of time for him, and he prefers to start working in other tasks that he knows he can start working and has enough information

What I understand so far is that the team should not start with tasks belonging to a different sprint because it would lead to loose the focus on the current sprint goal, the whole team should be involved on getting the work done as a team and not as separate members. We're not playing tetris trying to accommodate tasks as if they were shapes that could be arranged, we're delivering a specific set of functionalities.

I want to convince them, but I'm running out of arguments and I need help to get the team back in focus.

Am I wrong about it? Any advice?