Not complicated
[i do not want to restore my iphone to te newest update, because it would make my phone incapable of jailbreaking, but i have an appointment with the iStore, so i need my Cydia to temporarily not by on my iPhone, as to not void my warranty]
To remove Cydia from your iPhone, simply go into the 'Manage' in your Cydia store,
Packages
( make sure the top right of your screen confirms you're in "expert", instead of "simple")
And delete the "Cydia installer"!
Or just start deleting from the top, one by one, if you feel like reatarting your springboard a million times.
***BUT** once u delete Cydia, you will not be able to delete SOME of it's components (example: "Manual correct pro", "iNo Rotate", "Fastcopy". so delete the settin option guys first)
this does not unjailbreak your phone, but i notice IMMEDIATE speed after doing this, since it removed my winterboard/themes.
The only true ways of removing a jailbreak is by updating to the newest software in iTunes, or restoring your phone, and reselecting the music/photos/apps and such that you want to sync your device to.