I love the CRM forums because lot of people are asking very interesting question...
The last I liked was: how to remove some activities when I click on "New" button in the activities grid or when I click to the top menu "New activity".
Indeed, if you don't want to use the campaign response activity, or any other of them, you can't hide them in a native and supported way.
So, I give you some hints to hide these activities.
Please be aware that these methods are unsupported! Make a backup of each mentionned file before follwoing this procedure.
Removing an activity in the "New" activity dialog in activities grid
To remove the activity you want, open the file
Search for the block in blue in the screenshot below. You can comment or delete the lines that correspond to the activities you need to hide. Then save the file.
Removing an activity from the "New activity" top menu
For this one, open the file
Search for the function "CheckAddin()". This method is called during page loading. Add the line (see screenshot below) corresponding to the activity you need to hide.
The all list of activities id is: