You can control sync activities in your Admin panel. If you navigate to the salesforce icon in the admin panel it will give you access to the sync activities. If you find that there is too much data being written into the activities history you can limit those activities (form fills, email activity etc.). That might help with the volume of data that is rolling up into the account record.
Alternatively what you might try doing is altering the roll up in Salesforce only to show sales related activities at the account level. Right now our Marketo sync is controlled by a generic user "Admin" so we could filter out the activities that way (although it may take a bit of work on the backend). You'd lose out however on the more valuable Marketo data (ie form completes) in this scenario.