This is fairly common once a site starts gaining traction, and the easiest method to minimise spam is by activating a plugin.
Activate the plugin “Akismet” which should already be installed in your plugins area of the WP admin. You will need an API key from Akismet which can be obtained here – https://akismet.com/signup/. If you choose personal it is free, but business users are charged a fee, select what you think is appropriate for your site.
There are also other free spam plugins available such as: