Prevent Spam users registration

Out of 3 network sites I’ve, one has the role of registered user as author and rest two has subscriber.

In last a few weeks I’ve been getting a lot of spam users generated automatically in my instance and since the user has author profile in one network site, they are able to generate random posts as well.

I tried to watch out a lot of spam control plugins. However, posting this thread to ask:

1. Should I use any plugin
2. or, can it be controlled by any such coding?

If I’ve to use plugin, what are the recommendations of the experts out there?