There is an out of the box way to bulk delete members.
The first challenge you have is that it is not possible to delete members that have content, so, as an AGOL Administrator, you would first have to either delete a member's content or have it moved to another member. Then you can delete that member.
If none of the users have any content then you could create something using the REST API:
The workflow would need to get a token first and issue the delete requests as an administrator user:
Hi Fran, also worth looking at doing this with the API for Python, as it is makes it very easy to automate all sorts of processes in AGOL/Portal. Accessing and managing users | ArcGIS for Developers
You can even quite easily check to see if users have content, and if so reassign to another user etc, which you can see in the examples on the page I linked.
Hi Fran, the GeoJobe AdminTools, even the free standard version, allows you to do this. You have to choose what is to happen with content and groups, but you can plan that.
First, though, revoke any premium app licenses -- Community Analyst, Pro, etc.Get than done, then take a quick inspection of content and decide what to do, then you can engage the tool more successfully.