When you create a group in the admin panel, you can uncheck the "Enabled
Allow unrestricted cluster creation" option in the Entitlements tab.
This is pretty self explanatory.
However this does not mean no clusters can be created, because there is also something like Cluster Policies (defined on the Compute tab).
If your user group is assigned a cluster policy, the users from that group fall under the permissions of that cluster policy.
By default the "Personal Compute" policy is assigned to the All Users group. So you will have to change the policy accordingly (or create new policies).
So basically: disable cluster creation in the group and check cluster policies.