I'm sure you hate those ads poping up in free apps. I know developers are getting money from ads for maintaining the free app, but I don't care, I just hate those ads. For rooted devices AdFree is a great decision, but for unrooted there's no such a workaround. Hosts file can't be edited. But I figured out a workaround, which is not a complete solution, but can be useful if you're using your home WiFi. Note: this will only filter ads at home while using WiFi.
You need a wireless router, that supports access control. Mine does, it's a TP-LINK TL-WR1043ND. If yours doesn't support it, then it's a waste of time to countinue reading. I'm sorry.
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Step 1: Setting Up a Host
Go to your router's configuration page. In my instructable I'm demonstrating on a TP-LINK device, but other manufacturers make similiar menu, so you could try to follow. For TP-LINK routers it's 192.168.1.1. I think both the default password and username are admin. After logged in on the left select Acces Control, and under it select Host. Select Add New.... There from the drop-down menu you can select IP address or MAC address. In my case I'll use MAC address, because my internal IP changes, and it wouldn't work that way. My router menu has a help on the right, it can make things easier. For Host Description I use Galaxy. It's not important.
Step 2: Setting Up a Target
Now on the left select Target, then Add New.... I know, that Google's ad service is called GoogleAdSyndication so I'll use this keyword (I've just typed googlead as it's shorter) after selecting Domain name instead of IP address. In my case I could add max. 4 target keywords or domain names. For Target description I used AdMob, but it could be anything.
Step 3: Setting Up a Rule
On the left select Rule. Select Add New.... Make sure to select in every drop-down menu the correct item. Action should be Deny, and Status be Enabled.
Save it, and you're done. Now tick the Enable Access Control box, and select Allow below it. Then save it!
If you made all the things right, no more ads will appear at home ;)