First, you don’t mention if this is your own gitlab server, or are you using gitlab.com?
If it is your own server, then someone must have an admin login to it if they created it. Or perhaps someone else other than you in your company manages your gitlab server, so then you would need to contact that admin to disable it.
But I assume it’s more likely that you are using gitlab.com and you are owner of a repo here. So in this instance, no you do not have admin abilities to delete or block his account. In the project though, you do have a delete button next to their username (see attached screenshot). Next to testuser on a test project I created, there is a delete button. You can also change their access from maintainer to reporter which will limit what they can do. As a reporter, they will have minimal access to the project and repositories, you will need to check for both the project/group and also repositories inside the group. You could even set them to guest. Alternatively, click the red bin icon and remove them from the project.