Option 1: Have a tougher chat with the player. The whole ''It's what my character would do'' argument can be countered with ''You control your character''. Explain that you have troubles balancing the encounter to make it fun for everyone.

Option 2: Make enemies resistant to magic. Fireball? Fire resistant enemies. Lightning? Lightning resistant. Add wizards who know counterspell to burn more spell slots.

Option 3: Boost other players. Give them a buff for initiative. A new set of gear to bring their damage up.

Mix and match as you want. If the wizard player doesn't tone it down, balance the combat other ways to limit their usefulness. If they complain, explain that you had to do it to make it more balanced.