At the 'Options' menu go to the 'Code Entry' screen and enter the following codes to unlock the corresponding effect.

Unlocks Iron Man/Bonus costume(s):
Enter the code 111111

Unlocks Hulk/Bonus costume(s):
Enter the code 222222

Unlocks Wolverine/Bonus costume(s):
Enter the code 333333

Unlocks Thor/Bonus costume(s):
Enter the code 444444

Unlocks Silver Surfer/Bonus costume(s):
Enter the code 555555

Unlocks Falcon/Bonus costume(s):
Enter the code 666666

Unlocks 'Super knockback' Cheat:
Enter the code 777777

Unlocks 'No Blocking' Cheat:
Enter the code 888888

Unlocks Dr. Doom/Bonus costume(s):
Enter the code 999999

Unlocks Captain America and US Agent:
Enter the code 925627

Unlocks Spider-Man and Symbiote Spider-Man:
Enter the code 925678