Just vary the encounters a bit. There's nothing wrong with the magic-users blasting the hordes now and then. After all, they shine in crowd control situations. But make sure that there are a couple of higher level folks involved and that all encounters don't play out as one big horde charging the party. Get creative. Get wild. Get... TWO hordes! 
Seriously though, just change things up.
- Instead of charging in a single mob, charge from multiple directions.
- Instead of charging, skirmish (spread out say 10-15' apart) and use ranged attacks.
- Include leaders who can withstand a couple of fireballs.
- Have a group of orcs led by a Flesh Golem charge the party, while a skirmish line of 5 or 6 archers, backed up by the witch doctor, pins the party down.
- Use terrain to limit the effectiveness of AoE spells (heavy woods, twisting dungeon corridors, small rooms with doors, rock formations, etc)
- Use sneaky opponents and ambushers who close before the magic-users can respond
There are plenty of ways to ensure that combat is varied and fun for all. AoE spells are at their most effective in open spaces at range against hordes of low-level enemies. If you keep throwing those kinds of encounters against the party, it stands to reason AoE spells will remain the most effective tactic.

Seriously though, just change things up.
- Instead of charging in a single mob, charge from multiple directions.
- Instead of charging, skirmish (spread out say 10-15' apart) and use ranged attacks.
- Include leaders who can withstand a couple of fireballs.
- Have a group of orcs led by a Flesh Golem charge the party, while a skirmish line of 5 or 6 archers, backed up by the witch doctor, pins the party down.
- Use terrain to limit the effectiveness of AoE spells (heavy woods, twisting dungeon corridors, small rooms with doors, rock formations, etc)
- Use sneaky opponents and ambushers who close before the magic-users can respond
There are plenty of ways to ensure that combat is varied and fun for all. AoE spells are at their most effective in open spaces at range against hordes of low-level enemies. If you keep throwing those kinds of encounters against the party, it stands to reason AoE spells will remain the most effective tactic.