I found a house I'm interested in but the seller's agent won't present my offer to the seller.

The seller's agent is obligated to present ANY offer to the seller up to the day of closing. It's NJ real estate law. She cannot refuse even if there are better offers on the table or even under contract. Your agent is correct to ask for a written rejection. I ALWAYS make my sellers sign the contract as "rejected" so the other agent knows it was presented. You or your agent could report the seller's agent to the Board ethics committee but at a minimum her Broker should be contacted so he/she can verify that the offer has been presented. Good luck to you.