Delete things from $PATH
If you just typed in the  exports into the terminal as noted above then the variables in the environment will only last until you exit that terminal session. So simply typing exit and then reopening another terminal will remove any setting that you applied with the noted exports performed in your question.

export MAGICK_HOME="/Library/ImageMagick-6.7.5/"
export PATH="$MAGICK_HOME/bin:$PATH"
export DYLD_LIBRARY_PATH="$MAGICK_HOME/lib/"


Additionally you can un set variables from the environment by using the unset command.

For example:

unset MAGICK_HOME # will remove MAGICK_HOME from the environment. 


You can do it for the others as well but if you do it for PATH you will lose your path setting until you close this terminal session and reopen a new window,etc.

As an experiment try typing:  

env | grep MAGICK_HOME


You should see MAGICK_HOME=/Library/ImageMagick-6.7.5/ echoed back if its still in the environment. If its not then the settings have already been lost by a windows close. etc re-enter your export of MAGICK_HOME and try the above command again.

then 

exit


and then finally re open a new terminal window and enter:

env | grep MAGICK_HOME 


Your settings should be gone for MAGICK_HOME. The same is true for the modification done to all the other environment variables set with the export command right in a terminal session. They will just return to their default values if you have not set them in a start-up invocation file as required for your shell environment variables to have a more permanent place in your shell enviorment.