CustomVillageTrades is a plugin for Minecraft servers running Spigot. It changes how villagers make their trade lists, meaning you can make them trade whatever you like. Whether that means seeds for spawn eggs, a more expensive diamond, or no trades at all is entirely your choice.
Tab over to the releases page for the latest version. Download the jar file and place it in your server's /plugins folder.
The wiki has all you need to know about using the plugin. It will show you how to modify the trade trees, create enchanted items, or erase all trades.
If you know a thing or two about Java, Spigot, or development in general, you can fork the repository and make some changes. The issues page might give you an idea for what to change, add, or fix.
Testing is also sorely needed! Download the plugin, put it on your server, and tell me how it works or if it breaks. Remember to include the stack trace, what you expected to happen, and what happened instead, if you make a bug report. All bug reports and suggestions should be made either on the issues page or in the Spigot thread.