Hi @Fabrizio Nardi ,
If the third party could be shown in the WSUS console, we could decline them and then they could be marked as decline on the console of SCCM. If not, it maybe difficult to decline them if they have been synchronized.
Here is the article we could refer to:
https://www.reddit.com/r/SCCM/comments/9h6psu/configmgr_1806_thirdparty_updates_expire_updates/
Note: the above links are not from MS, and just for your reference.
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