How NOT to redirect using the Rules module?
A possible "remedy" for this is to use the Rules module (indeed ...). To do so, have a look at my answer to "How to restrict access to a node via node/12 and allow access via a path like content/sometitle?".

Use the rule included there (within "Option 1"), and apply these changes to it:


Adapt the regular expression within the "Rules Condition", to make it match your (user/[uid]/comments.).
Adapt the "no_access" url to your user/[uid].
Probably you don't want to display any message, so just remove that "Rules Action".