It sounds like you are modifying an existing project which has camera movement or has a motion clip which has the head moving forward and backwards. Try starting a new project and then load the character you want to use from the Actor tab and not the Project tab. This will not have any zooming in or out at all.
However, if you want to use a particular project, you can try opening the Timeline and then delete all the keys on the Transform track. This will remove all camera movement. Unfortunately, if the head movement is in the Motion Clip then you will need to remove the Motion Clip itself from the Timeline.
If you continue to have problems, please tell us which project you are using and I can advise you further.