There is a word play.
Но на происки и бредни
—For intrigues and ravings
.
Сети есть у нас, и бредни
—We have nets and drag-nets
.
In the first line бредни
(from бред
— delirium) means silly twaddle, ravings, while in the second line it's about fisherman tackles: сеть (net) and бредень (drag-net), which figurative can be used by authors of the letter to fight with that fraud.