You are unable to remove the third levels reward from the config. Both when the server is down and when editing the config while the server is live with /ksreload. It always comes back.

levels:
  '3':
    enabled: false
    announcement: '[displayName] &cis non-stop killing!'
  '1':
    enabled: true
    announcement: 'example'
    commands:
    - example

KillStreaks v1.3.0
Paper dev 1449 (Spigot 1.12.2)

killAbuse:
  enabled: true
  maxKills: 1
  duration: 1800

You are able to kill the same person over and over again to get the reward. It still announces the kill and the streak rises.

Reset a player's killstreak via a command. Support UUIDs and names.

Would it be possible to add an option to reset the killstreak rewards after you have reached the end? For example, go back to 1 after reaching the maximum of 10?

Alternatively, would it be possible to have a reward for every kill in addition to the killstreak rewards so you can still get a reward when your streak is maxed out?

Apparently you are able to kill yourself and get the rewards configured for kills.