You can turn admission controllers on and off in your server with two different commands.  Depending on how your server was configured and how you started kube-apiserver, you may need to make changes against systemd, or against a manifest that you created to start up the API server in the first place. 

Generally, to enable the server, you'll execute the following:

kube-apiserver --enable-admission-plugins

And to disable it, you'll change that to the following:

kube-apiserver --disable-admission-plugins=

If you're running Kubernetes 1.10 or later, there is a set of recommended admission controllers for you. You can enable them with the following:

kube-apiserver --enable-admission-plugins=NamespaceLifecycle,LimitRanger,ServiceAccount,DefaultStorageClass,DefaultTolerationSeconds,MutatingAdmissionWebhook,ValidatingAdmissionWebhook,ResourceQuota

In earlier version of Kubernetes, there weren't separate concepts of mutating and validating, so you'll have to read the documentation to understand the implication of using admission controllers on earlier versions of the software.