Well, you can stop already created Salesforce records from syncing, if you have setup a filter field for that. Such a field would need to be created in Salesforce and can only be controlled from Salesforce.
But I guess what you want to do in the first place is prevent records from being inserted into Salesforce. And that insertion process should better not be all over the place, but ideally only in one "Sync to Salesforce" campaign.
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