First, banning IP’s is generally not a good idea. Bittorrent’s own algorithms will automatically seek out the best connections and avoid the worst ones. You shouldn’t need to ever interfere in this process, since it has better information that you do and reacts faster than you can. Let it manage its own network.
Next thing is your upload speed. You should limit it, yes, but you’re probably overdoing it. You need to test your actual upstream bandwidth on one of the many websites available for that purpose. Follow the instructions carefully to do the test. Keep your units straight when converting for BitComet. Set your limit to about 80% of your actual, tested upstream rate and you should be fine.
This may be a contributing factor to your slowdown. If you set your upload rate too low, you aren’t a good trading partner for other swarm members, so they will tend to gradually snub you in favor of others with higher upload rates.
But there can be several other causes. Some, you can’t do anything about. First, let’s check your configuration. Make sure that you have at least some peers showing remote initiation. If you have nothing but local and NAT peers, your listen port has been blocked and you’ll need to fix that. If you do have remote peers, it’s golden.
Next, look at the other peers in the swarm. If you have, e.g., 78% of the file, and all of the other peers also have 78% or less, then what’s happened is you’ve caught up with the swarm and will just have to accept that the rest of it’s going to be a lot slower. This happens when you’ve got one seed for a large torrent. Those who join the swarm later get most of their pieces from other swarm members, but they all eventually get caught up with the first joiners. Then it slows down to the rate the seeder can distribute the rest at.
Make sure you do have at least one seeder in the swarm, even if you’re not connected to it yourself. If there are none, then you’ll just have to hope someone will start seeding again soon. How long you want to wait before you give up depends on too many things, and is largely up to you anyway. If there’s a forum or comment thread on the torrent, check it to see if there’s any news.
I should add, don’t give up immedately. Some forms of inital-seeding will indicate that there are no seeds, but the swarm will make consistent progress. For those, just be patient and it should get there.