Not sure if there exists a mechanism within Osclass to achieve this but you can try the following plugins:
--- There does exist a mechanism in oslclass to mark as spam/ block, but i have no idea how this works, as duplicate ads even with the exact same title and description is never marked as spam.
First One Plugin 1.3.3
Gives the users the freedom to push their Ads to the top. This way they will have fewer tendencies to post duplicates
--- This plugin has nothing to do with spam, this is a plugin to move an item to the top.
Activate Deactivate Item 3.0.1
This is useful for those who want to pause their Ads for certain duration and reactivate when needed.s
--- This plugin is for activating and deactivating ads. Again, nothing to do with spam.
The only solution i see is the plugin by Michael (https://market.osclass.org/plugins/security/spam-solution-plugin_318
) . But i still don't get what is the spam/block inbuild function in the osclass core. And why doesn't it work for such a basic feature.