I'm assuming you have the entire google play service as a dependency in your app.
From Google Play Services version 6.5 and beyond you can select which
individual APIs you want to use, and import just those ones.
This is the entire list
Google+ com.google.android.gms:play-services-plus:6.5.+
Google Account Login com.google.android.gms:play-services-identity:6.5.+
Google Activity Recognition com.google.android.gms:play-services-location:6.5.+
Google App Indexing com.google.android.gms:play-services-appindexing:6.5.+
Google Cast com.google.android.gms:play-services-cast:6.5.+
Google Drive com.google.android.gms:play-services-drive:6.5.+
Google Fit com.google.android.gms:play-services-fitness:6.5.+
Google Maps com.google.android.gms:play-services-maps:6.5.+
Google Mobile Ads com.google.android.gms:play-services-ads:6.5.+
Google Panorama Viewer com.google.android.gms:play-services-panorama:6.5.+
Google Play Game services com.google.android.gms:play-services-games:6.5.+
Google Wallet com.google.android.gms:play-services-wallet:6.5.+
Android Wear com.google.android.gms:play-services-wearable:6.5.+
Google Actions
Google Analytics
Google Cloud Messaging com.google.android.gms:play-services-base:6.5.+
Source of the list
So i guess if you add these after remove com.google.android.gms:play-services-ads
(and others that you dont need) instead of com.google.android.gms:play-services
(the whole bunch) you should be able to get rid of the admob related stuff (and other unwanted stuff).