userres table does nothing about resource (CPU/NET) allocation and consumption. It depends on set_resource_limits only, and userres or delband manage staked or delegated amount of EOS and make it available to utilize this information from eosio and other accounts' contract.
I'm not sure how I can help you best. It is difficult without knowing the contracts code.
In any case you can delete data/multinindex-rows with the multiindex-erase function.
But i see a problem in the design of the contract. A resource saving contract should always avoid saving strings, but from what I see, you are saving three, per row.
To save additional ...