What I've done in the past is added a filter to the smart list portion of my SFDC trigger campaigns. So if my campaign triggered an SFDC sync at a score of 100 or greater, I would create a list called "Don't Sync" and add in a "Member of list IS NOT" filter to that trigger campaign.
Then I would upload my tradeshow leads to that list and go from there.
Just make sure you know all the trigger campaigns that could possible insert the lead into SFDC and add this filter to all of them.