Is it a private tracker?
If yes, all you need to do is to leave the task seeding for whatever minimum time they request. You can’t control how much you upload, since on private trackers there aren’t as many peers as on public ones, therefore often you will just have keep the task seeding for a while (a few days minimum).
But that’s more clearly specified in the particular seeding rules, for each private tracker site. You’d have to check those, to make sure that you comply.
However, if you were banned from a specific tracker, I don’t think that you would be able to download anymore, at all. Your passkey wouldn’t work anymore on the tracker probably therefore your connection to the tracker would be refused altogether. If you’re still downloading but at slow speeds, the cause is probably another one.
Depending on your connection’s upload capabilities, uploading at 10kB/s (not kb/s) isn’t at all bad. This is one of the very reasons why most people need to leave the tasks seeding for a longer time after the download.
Most residential connections (xDSL, cable) have a much higher download bandwidth than the upload one. This means that you will be able to download a 700MB task in, let’s say, 40 minutes, but it will take you much longer to seed back to a 1:1 ratio (i.e. upload 700MB).
As long as you’re seeding, and you don’t intentionally limit too low your overall upload capabilities, you needn’t worry. Nobody is judging you for not having a killer upload speed. As long as you keep seeding the task you’re doing your part for the community; you just need to leave it seeding until it hits at least an 1.5-2.0 share ratio and you’ll be more than fine.
A bandwidth of about 8-10kB/s upload per seeding task is considered acceptable (if you seed multiple tasks) for public torrents (where your peers are actually downloading most of the time from you). So you should seed simultaneously no more than 2-3 taks, so that your overall tested upload bandwidth divided by the number of seeding tasks doesn’t fall under the minimum of 8kB/s (unless of course you have a high upload bandwidth and you can afford to seed more tasks simultaneously without spreading too thin the individual bandwidth for each task). The other ones you can queue for seeding using the options page of BitComet.
You should also take into account that actively downloading tasks also need upload bandwidth in order to download (they have to upload the pieces they already have for that torrent in order to receive other pieces from their peers). Therefore you should also include those in the calculation when dividing the overall upload bandwidth, especially since BitComet prioritizes the downloading tasks over the seeding ones and thus the upload bandwidth will be allocated prioritarily to the downloading ones when needed.
As a thumb rule, the more upload bandwidth you allocate for a downloading task, the more speed you may get (no one can guarantee you that, as download speed depends on more factors than this, but you open the possibility for it). If you don’t have enough upload bandwidth for a task almost surely you won’t get any high speeds for it (with the notable exception when the task has lots of seeds but not many peers, and you still may download at acceptable speeds).
For private torrents OTOH you can keep as much hundreds of tasks seeding at the same time, since most of the time there will be nobody downloading from you for 99% of the tasks you’re seeding (but this will add up to your seeding time thus fulfilling your seeding quota for those tasks and/or earning you bonus points which you can trade in for upload credit, depending on the site’s policy).
Use this settings guide, to check and make sure that your speed related settings are in order.
If it’s a public tracker you’re talking about and your listening port is open, well, then if you’re not uploading at all it could mean that there is nobody who wants to download that resource for the time being (although that’s rarely the case for a torrent that’s still alive). You don’t need to do anything more than leaving the task seeding and making sure that you have an open listen port so that other peers can contact your client and initiate a download from it.