Is there an existing module to bulk delete user content but not user?
I'd suggest to try out Rules module for this, you've got an event: "User has logout", grab the entities from that user, loop them and delete them.

Here's a exported rule that would do it (change the number of entities to suit your needs)

{ "rules_test_logout" : {
"LABEL" : "test_logout",
"PLUGIN" : "reaction rule",
"REQUIRES" : [ "rules" ],
"ON" : [ "user_logout" ],
"DO" : [
  { "entity_query" : {
      "USING" : { "type" : "node", "property" : "author", "value" : [ "account" ] },
      "PROVIDE" : { "entity_fetched" : { "entity_fetched" : "Fetched entity" } }
    }
  },
  { "LOOP" : {
      "USING" : { "list" : [ "entity-fetched" ] },
      "ITEM" : { "list_item" : "Current list item" },
      "DO" : [ { "entity_delete" : { "data" : [ "list-item" ] } } ]
    }
  }
]


}
}