If you are a WordPress user self hosted, I am sure you should have installed many plugins and after deactivated most of them and after that deleted many of them. If I am not going wrong again you should have installed some of plugins after deleting if you feel useful.

After installing and activating the plugin you make some settings or configuration for that plugin. So if you deactivate and delete that plugin your settings does not get deleted by itself. This is the reason if you install that plugin again you will not have to configure it again. However it is not true for all plugins but for many of them. You have to delete it manually.

So be careful in this, phpMyAdmin contains all the database tables of all of your blogs so if you have more than one blog then make it clear which database is for the blog you want to edit. See following screenshot, there are five database for five different blog or websites. Once you are sure you can select a database

This process is not so simple you think. You have to go in phpMyAdmin panel to do it. Before making any changes it highly recommended to backup your database. So if any problem causes you can get back normal again.

Where is phpMyAdmin?
Login to your web-host where your website is hosted as my website is on Bluehost. Usually you will see cPanel or the interface you see is cPanel. There navigate to database section you will find phpMyAdmin.

When you install wordpress, it creates some basic database tables. That we know as mysql database. So when you install a new plugin that also creates some database. It can be a different database table or it can be inside a database table. If it does not create a separate table it will save its settings in wp_options. If you delete (drop) this table (wp_options) your website will go down or it will become inaccessible. Because this table contains all the plugin information, website name, contact email, blog description, RSS information all the basic settings on wordpress you see admin panel. If you think you have deleted a plugin but it has created a table that you can delete form phpMyAdmin. Once you have selected database there look for tables that you do not need or the plugin you have deleted but their data table is still here. You can see wp_thesis_terms in below screen shot because I have installed Thesis theme but if I am using another theme and still there is its data and adding extra load on my blog  so simply I can delete it if I am sure what I am doing. If you are done there look for  wp_options because it contains many of plugin data. See following screenshot.

If you are reached there click on the icon next to wp_options in action section. It will browse or open the wp_options, see following screenshot.

Now this is where you have to go but careful before deleting any ID, You can select all the tables but it is not recommended. Now it is time to see settings for deleted plugins. I had installed thesis openhook plugin and deleted it but all the setting I found here. All the option were starting with openhook_ and I deleted them all. I had installed Facebook Like plugin and all the option of that plugin were starting with fb_ and if you have installed tweetmeme plguin you will see all the tweetmeme settings are saved with tm_. So if you are 100% sure which ID is for which plugin and you do not want that you can delete it.

If you delete a setting that is not core WordPress setting your website will not go down otherwise your website will go down. Best of luck.