Hey Melanie, I found this old thread and I'm curious about how you set up the no-sync settings. From what I understand, you created those rules in Salesforce, correct? What does that look like?
You can use sharing rules or permissions in SFDC to hide leads/contacts from the sync user, which prevents them from syncing to Marketo.
You can also open a support case with Marketo to have a custom sync filter put in place. This would be a custom field in your SFDC instance like "Sync To Marketo". Any lead or contact marked with that field would be synced down to Marketo, and any others would be ignored. This should be a last resort though, as the sharing rules and permissions give you more control over your sync.
Yes I did create those rules in Salesforce - it is quite complicated and involves code.
How it works is my Salesforce Developer created some code and we have a custom object that (as a non salesforce developer) I can update and maintain and the code just reads my dataset records. Everytime a lead or contact is edited it triggers the code to check through the list of criteria.
e.g of a Dataset - IF Record type ID = xxxxxxx AND Email address <> NULL AND Invalid Email = FALSE then mark a checkbox "Marketable record"
Marketo Support created me a checkbox which we called "Push to Marketo?" so when that checkbox is true the record syncs. When it is false it breaks the connection.
I have a workflow that when Marketable Record is edited to true (via code) then this updates the Push to Marketo checkbox and the sync happens
I also have a time dependent workflow to do the reverse, so when Marketable record is marked as false (i allow an hour for all necessary updates to copy across) then the Push to Marketo checkbox is unchecked.
I do regular cleanups in Marketo to delete the records that have become unmarketable.
Gosh I hope that makes sense.