While composing a new message in Message and Reminder Center, There is only option for adding the recipients and not to remove them. It will be helpful to the end user for removing the accidentally added recipients list.
Kindly follow the step,
Messages -> Add New -> Select Users From The Drop down List. There is no option to remove the added user from the list.
Let us know if this feature can be added. If yes, we will work on it and share the code.