Spam on registration by free membership plan

There is a plan for membership and it is free.
It is created for people who paid outside the system. The link is send only to those who have paid.

Suddenly, llms was flooded with spam registration, orders. System became very-very slow.

I cleaned thousands of orders and transactions and users from the database. the system now work normal.

But i wish i can protect llms from such spam in the future.

I ve read some article about open registration and using the voucher system.

1. Is it for my case or i should use something else?
2. If i should use vouchers, i do not understand where users should type it: on the registration page which is opened by membership’s plan there is no field for voucher.

What i need at the end: i have some membership and i should be able to give existent and new users access to this membership for some period of time (2 months).