Can a threshold be set so that Tixati automatically removes or ignores a peer or seeder that contributes to a download less than a given # of valid bits / time interval or does not contribute at all?  

I realize the general and present attitude and implementation seeks to promote sharing, not restrict sharing.  Yet, there is an advent of torrent clients that seek only to identify the IPs of sharing participants.  Their purpose is to facilitate legal action against those who infringe on copyrighted material.

Typically such clients will not upload anything or will upload faulty data, for if they should participate in the share, that would compromise the plaintiff's case -- in some way, apparently.  Setting such a threshold would be a first step to impede such clients.

If such a threshold can be set, how to do it?

Tixati already blocks clients that upload faulty data. If you see those in the peer list ("Bad data") then you can block them manually in your block list and that will prevent them for future transfers.

If there are peers that don't upload, how can you be sure they are bad? They may be operating with a data cap and the torrent you are both connected to is unfortunately being blocked for outgoing that way. I see seeds as well as peers coming and going regularly, sometimes giving nothing for ages then they are very generous. Very few people upload/download one thing at a time or have fibre connections, and upload bandwidth is typically highly asymmetric... They may have a time schedule in their client for blocking/enabling bandwidth (perhaps while a home machine is being used for work or evening live streaming). Tixati should rotate out the least-uploading peers regularly, but you still have to connect to someone to ask if they will send anything to you.

While it sounds like a good idea, if you are aggressive about removing every connection (permanently) that doesn't upload much at a certain time, you might even end up blocking your only seed later, who has bandwidth spare only after other peers remove torrents without checking if anyone is still downloading from them (especially if that's done automatically by time or upload ratio).

I forgot to add to that (too late to edit): there is a case where Tixati could sanity check the connections for you. The fake peers you mention that never contribute, also often never appear to download. Their completion ratio stays at 0% - obvious if they aren't prepared to receive data on the grounds they would be breaking the law they are trying to uphold.

Tixati knows exactly which pieces a client has already downloaded and is requesting. It must get a map of the remote client's piece completion every time it requests something, because it then decides what to send based on the map of missing pieces and the policy you have set for seeding the torrent. That is also how it builds the "available copies" map from all remote peers on the Details tab of a torrent.

So if a fake peer requests data and the completion ratio never changes, and the piece data Tixati has sent never shows up in the peer's completion map, it should easily be possible to confirm that as a fake peer and block immediately (globally), just like a peer that sends bad data. That would also reduce resources connecting to fake peers that will never contribute, which should be a factor anyway in Tixati rotating out low-contributing connections in favour of others. To be clear, that's not the same as a client that never allows data connections (remote choking), perhaps because the remote machine is unable to release enough resources - just those that accept the connection, request and take data, but it never shows up as received.

I'd definitely like to see that implemented, as it's trivial to do that sanity checking (given the amount of data Tixati is already collecting on remote peer completion) and protects users very effectively, while also helping to reduce Tixati's running footprint (memory/bandwidth) by blocking obviously fake connections.

To add to the great first answer
1) I can be a peer with a partial download or NOT SELECTED any files for download and sit at 0% until I decide to begin to download.
2) Sending own piece info is not mandatory (for leech-downloading). Super-seeding modifies this behavior too (not recommended because it may show to your swarm that zero complete seeds exist).
3) Tixati already allows you to prefer "trading" bandwidth as opposed to just seeding, it's next to bandwidth controls.
4) You should read these forums to add bad surveillance IPs to the IP Filter in Tixati

Let's call your idea of "scientific interest" because it'd be practically impossible to implement something as this correctly, because Bittorrent doesn't behave as transparently as you think.