I would use my arms to defend myself
I somewhat expected this answer and was about to write about that but I thought it was too trivial, but in hindsight I should have mentioned it.
So in my example, you simply
do not have ways to defend against method 2. Period.
The only way to really defend is to drastically change the transaction format, to something like Grin's. This would come with a lot of side effects.
To sum up: there is a plan for non-spammers, who want to have a payment system. For everyone else, there is spamchain. You, as a user, can run any software you want. It is your choice.
Which is your plan, run Knots?
Again, you would then incentive the more harmful spam, the "fake public keys" spam.