I have "subscribe by email" for subscribing to my blog. My

I have "subscribe by email" for subscribing to my blog. My theme "simplemarket" also has "users". I obviously need "admin" as a user but would like to prevent the spammers from becoming users and leaving their spam messages.

I think what I want is to block anyone other than me from adding "users" but I want to allow people to subscribe to my blog and also to be able to leave comments on my posts. I think probably that commentors should have to register as subscribers.

The "subscribe via email" plugin works nicely, but is there a way I can block everyone else from adding users without stopping the "subscribe by email"?