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I'm not quite sure what you mean by "shadowban", but this does pretty much exist already.

Most communities on the network - I believe all of them except for Electrical Engineering at the moment - currently have "new site mode" enabled, which grants all new users the Participate Everywhere ability, which removes certain new-user rate limits.  
If a community decides that the time has come to turn "new site mode" off, the Codidact Team can do that pretty easily as long as that's what that community decides. That means new users would be granted the "Participate" ability and not "Participate Everywhere" immediately upon joining the community.

Currently, the Participate ability allows you to post 3 top-level posts (questions, articles) and 20 second-level posts (answers) within 24 hours. The Participate Everywhere ability significantly raises those rate limits.

Moderators on individual communities can suspend or remove individual abilities from specific users. If someone is abusing the Participate Everywhere ability, a moderator can remove it. If someone is abusing the Participate ability, a moderator can suspend that ability. Moderators can also issue broader suspensions for individual users. (Ban evasion is a legitimate reason to issue a suspension.)  
This is usually the best way to handle a disruptive individual, and doesn't require the intervention of the Codidact Team - community moderators have the tools to handle that at their disposal.

The Codidact Team does have some stronger tools available, such as STAT, and we're around if the mods need us to step in (such as through Discord, through flag escalations, or just noticing what's going on).

The rate limit values for both users with Participate and Participate Everywhere can also be adjusted on a per-community basis:

![Four values in the administrator tools, displaying the number of top- and second-level posts that can be posted by both new and established users](https://meta.codidact.com/uploads/jxbu3k8jsc5g8wxw5yeklwruo90z)

However, I don't think it would be a good idea to start fiddling with those values because of a single disruptive user. We have other tools more appropriate for handling that, and you have access to the most basic and yet most important one: Flagging.  
If you spot an issue, flag it. If you think someone's evading STAT or some other block, raise a flag. The community moderators will see the flag, and likely the Codidact Team as well. But if nobody is flagging suspicious behavior, it's much more likely to slip under the radar.