My favorites:

Proof by cosmology

The negation of the proposition is unimaginable or meaningless. Popular for proofs of the existence of God.

Proof by semantic shift

Some standard but inconvenient definitions are changed for the statement of the result.

Proof by religious belief

God told us that X is true

Proof by parent

“Because I said so.”

Proof by slippery definitions

The definitions are not so well-defined that the reader can tell exactly what they mean.

Proof by eminent authority

“I saw Karp in the elevator and he said it was probably NP-complete.”

David Karp?