Listen,
#1 - The MECH is a BAD add-on. Bad for the computer, the server, the player.
Nothing against it, cool idea, just very evil. Like the devil... Mechs worse.
#2 - To delete it, go to Blockland in program files, (the folder...) and then proceed to add-ons. Find the mech zip and delete into your recycle bin. Nothing overly dramatic like hiring a hitman... just kill the file.
You should not worry about having TOO many files on unless your computer is slow... then it may be the music thats the problem. The addons have no major space issues other than music, since it is the only option you can choose to NOT download :P