How to prevent heroes running past my defenses?
There are usually about three things you can do to slow rebels down:


Big damage, forcing them to rest - the most obvious way, but very situational if it will work enough to slow them down for a specific round or objective (though it is always good, the closer to wounding you get them the better—it just may not be enough if your one goal is objective positioning denial)
Stun - Stuns (and other Harmful conditions, but stuns are the most effective) reduce their action choices and uses, which is a big help
Figure blocking - get your figures in their way (most heroes have to use extra movement points to go through you) or park them on/around the objective you wish to deny (they can't stop their movement temporarily on you to perform an Interact); lots of small figures in one deployment card (Troopers for example) or Large figures work well for this, but you get enough of anything and it can get the job done


Personally, I would generally only use the third option as a last resort. For one, it jams you up so it's more difficult to attack because of a lack of line of sight for the figures in the back of your denial pile. But mostly because it's not too fun to play with nor against that tactic. But it is a viable tactic nonetheless.