Depending on the reason behind deleting a user, there are a few different actions you can take.
If you would like to terminate a user's access to your private wiki because they are no longer entitled to privileged information, you can either block them using Special:BlockUser, or you can install the UserMerge extension (formerly called User Merge and Delete), which allows you to truly delete the user. Blocking a user may prevent them from editing, but the user is still present. If you want to completely deactivate their ability to log in, we recommend you use UserMerge.
To install the UserMerge extension, go to the Self Service portal and install it to your wiki. If your wiki isn't supported by Self Service, open a support ticket and we will install the extension for you.
If only a few users are spamming on your wiki, you can choose to block them. However, if there is a massive spam attack on your wiki, please open an emergency priority support ticket and let us know so we can take care of it immediately.