To enable IP filtering (Blocklist) in Transmission:

1. Open Transmision

2. Go to Edit>Preferrences>Privacy

3. Enable Blocklist

4. Copy and Paste link below:

http://list.iblocklist.com/?list=bt_level1&fileformat=p2p&archiveformat=gz

5. Hit Update, wait and it's done.

You can mark Automatic Updates to make your life easier.

PS: This is a Level1 blocklist, it might happened that when the program is running it might stop you from going to some websites- bear that in mind.