diff --git a/cylc/flow/cfgspec/globalcfg.py b/cylc/flow/cfgspec/globalcfg.py
index 4988da885e2..46ed582f10b 100644
--- a/cylc/flow/cfgspec/globalcfg.py
+++ b/cylc/flow/cfgspec/globalcfg.py
@@ -48,6 +48,7 @@
     Range,
     cylc_config_validate,
 )
+from cylc.flow.run_modes import JOBLESS_MODES
 
 
 PLATFORM_REGEX_TEXT = '''
@@ -1887,6 +1888,13 @@ def default_for(
 def upg(cfg, descr):
     """Upgrader."""
     u = upgrader(cfg, descr)
+    for platform in JOBLESS_MODES:
+        u.forbid(
+            ['platforms', platform],
+            GlobalConfigError,
+            f'Platform name "{platform}" is not permitted.',
+            is_section=True,
+        )
     u.upgrade()
 
 
diff --git a/cylc/flow/parsec/upgrade.py b/cylc/flow/parsec/upgrade.py
index 3ef53b006f6..8e189d87bda 100644
--- a/cylc/flow/parsec/upgrade.py
+++ b/cylc/flow/parsec/upgrade.py
@@ -80,7 +80,13 @@ def deprecate(
                 'silent': silent, 'is_section': is_section
             })
 
-    def obsolete(self, vn, oldkeys, silent=False, is_section=False):
+    def obsolete(
+        self,
+        vn,
+        oldkeys,
+        silent: bool = False,
+        is_section: bool = False,
+    ):
         """Remove an obsolete key from a config
         Args:
             vn (str):
@@ -89,18 +95,57 @@ def obsolete(self, vn, oldkeys, silent=False, is_section=False):
                 Path within config to be removed.
             silent:
                 Set silent mode for this upgrade.
-            is_section (bool):
+            is_section:
                 Is a section heading.
+            hard_error:
+                Raise an error _NOW_.
         """
         if vn not in self.upgrades:
             self.upgrades[vn] = []
-        cvtr = converter(lambda x: x, "DELETED (OBSOLETE)")  # identity
+        else:
+            cvtr = converter(lambda x: x, "DELETED (OBSOLETE)")  # identity
         self.upgrades[vn].append(
             {
                 'old': oldkeys, 'new': None, 'cvt': cvtr, 'silent': silent,
                 'is_section': is_section
             })
 
+    def forbid(
+        self,
+        oldkeys,
+        error: Exception,
+        message: str,
+        is_section: bool = False,
+    ):
+        """Disallow a subset of arguments.
+        Args:
+            vn (str):
+                Version at which this obsoletion occurs.
+            oldkeys (list):
+                Path within config to be removed.
+            silent:
+                Set silent mode for this upgrade.
+            is_section:
+                Is a section heading.
+            hard_error:
+                Raise an error _NOW_.
+        """
+        if 'forbidden' not in self.upgrades:
+            self.upgrades['forbidden'] = []
+
+        def callback(_):
+            raise error(message)
+
+        self.upgrades['forbidden'].append(
+            {
+                'old': oldkeys,
+                'new': None,
+                'cvt': converter(callback, ''),
+                'silent': False,
+                'is_section': is_section,
+            }
+        )
+
     def get_item(self, keys):
         item = self.cfg
         for key in keys: