mmF001 Automatic Regeneration:

Prerequisites: Regeneration
Choose a character. Once, at the beginning of your turn, this character uses Regeneration as a free action.

Leech's trait:

POWER NEGATION: Other characters within 6 squares can't be given power actions.

Character "A" has regen and wants to use Auto regen to heal. Leech is within 6 squares of character "A". Does Leech prevent character "A" from using Auto regen?

No. Using regen with auto regen is not a power action.

No. The rule in the pg says that a specifically named standard power given as a free action ceases to be its native action type.

"You can give the character a power action as a free action" is still a power action.
"You can use (whatever) as a free action" is a free action.
Leech would prevent the former, but not the latter.