I -repped somebody and showed proof that they were a scammer and it was removed

[Excentric]

I really don't get how someone scamming isn't a valid reason for a -rep, and they didn't get steamrep tagged either, i have no idea why it was removed, and the automated response doesn't exactly give a reason

[D.Alex]

If you have evidence you're supposed to make a report by clicking red flag icon on user's BPTF profile page.

 

Only a report (on BPTF or SteamRep) could possibly lead to a SteamRep ban.  When you leave -trust it only stays until contested.

 

Even if your report is unsuccessful advanced users can still find it and make a decision by themselves (example).