using (ClientContext clientContext = new ClientContext(SiteUrls))
{
Web oWeb = clientContext.Web;

clientContext.Load(oWeb, w => w.RoleDefinitions);
clientContext.ExecuteQuery();

//CustomPermission1 is an existing custom permission level assigned to site owner group
RoleDefinition ContentControl = oWeb.RoleDefinitions.GetByName("CustomPermission1");

clientContext.Load(ContentControl, C => C.BasePermissions);
clientContext.ExecuteQuery();

ContentControl.BasePermissions.Clear(PermissionKind.ManagePermissions);

ContentControl.Update();
oWeb.Update();
clientContext.ExecuteQuery();
}