How to prevent my CPU from hitting 100%

I have a droplet running Ubuntu to support a small Rails app and a few weeks ago, my handful of users reported that they couldn’t get on, and I discovered that there was some bot indexing every page on my site, hitting several pages per second and using up all of my CPU capacity. This now seems to happen once per week. I am not a highly skilled web master - somebody set up the droplet for me and I just know how to write the Rails code and work with the database. Is there anything I can do on the server to, say, limit how many pages one user can hit at a time, or anything else that would prevent this from happening?