This means making sure the enemy team can’t capture the flag or cap a base. The best classes to do this would be melee with low-cost spammable AoE. Ideally, get one of the following to sit on top of the flag:
- Rogue: Fan of Knives
- Monk: Spinning Crane Kick
- Death Knight: Blood Boil
- Warrior: Thunder Clap
If someone else is spinning the flag and dies use your AoEs on the flag until someone offers to take over.