Actually you can only block the portal from beeing entered. So you propably need very fast units that can bypass oponents units and block the portals from beeing entered.You can stand on portal to block it.
The rules for leaving the portal are so forgiving, that you need a realy big unit to block the portal. Nearly impossible to do with a lot of armies.
Rules for leaving the portal should be way more restrictive (touching the portal on leaving) and also entering the portal after a quick reform should not be possible.