This is technically a Steam question, not really an Ubuntu question, but here goes!
Since Steam is a game manager, all installs and uninstalls can be done directly inside the Steam itself. Here are the steps to uninstall a game:
Open Steam and click on the big 'Library' button at the top of the window (you will see it in-between 'Store' and 'Community').
On the left side of your window, there will now be a list of the games you have purchased through Steam (including Free-to-play games). Just to be thorough, let's make sure we are looking only at the games you have currently installed on your machine by switching what is displayed of your Steam Library. Let your mouse/cursor hover over the big Library button you clicked on before. A pop-up menu should appear right underneath the word Library - click where it says 'Installed'. You can get the same pop-up menu by clicking the button directly beneath the big "Library" as shown below:
Now find the title you want to uninstall from the list shown on the left side of the menu, and right-click on its name. Then click on 'Delete Local Content'.
You will then be asked to confirm the process. Once Steam has finished, your game has been uninstalled.
Now, regarding the deletion of all files "related to Counter Strike Source, so that next time if I install it, it wouldn't keep the settings from previous installation" - this is not how Steam works. Steam is a game manager. In Linux you can do anything, and certainly you can remove any trace of Counter-Strike Source from your hard drive, but that is not the same as uninstalling it. At the end of the day, if you don't uninstall through Steam itself, there will still be a reference to the game inside Steam, and you will still have to do the process we have just gone through above to 'Delete Local Content' even if it is just to remove the reference to the game from inside Steam. And unfortunately as far as game settings go, those aren't always just stored locally on your machine, they are sometimes also stored on Steam servers (particularly for save game files of a Single-Player game - but this is a good thing!). However, I have never noticed a multiplayer Valve game that actually does this. Any time I re-install Counter-Strike or Team Fortress I have to go through the process of reconfiguring controls and display settings so I think you are completely safe.