Can I remove the male plants now?

Yes.

Can I leave the female ones in the ground to turn brown

Yes you should until fully dried and brown, Then pull them up and strip off the pods.

You can learn more about this over on the Seed Saving forum here. They have a great set of FAQs there on all sorts of seeds.

Dave

Here is a link that might be useful: Seed Saving Forum

Y'know, well-grown spinach seeds come from big, robust, widely spaced plants that have not been picked from and therefore produce really nice seeds. Saving seeds from randomly bolted spinach plants will not give the same results.

Thanks :). My spinach got big and bolted before I had a chance to pick from most of it. I planted a little too late. So I have big plants that were not picked from that I want to save the seed from. Same happened to my bok choy. I didn't pick any of that. It grew great in the fall but planting in the spring didn't work for me. It flowered and now has pods.