Communicate to your team to start camping the speed shrines and keep an eye on the back capper. Use pings to get your team to move with you (I know at low elo you often get these kids that berate you for "talking too much" or pinging, just calmly explain and stay positive).
Poor communication on the enemy team can lead you to 4v3 stomps allowing you to split push bot or mid and sending 2 more to roam.
If all of this proves too difficult, if you locked in a decently mobile champion you can chase and harass the back capper. For kassadin, your team is just going to have to land CC and shut him down. For teemo pick up a grez or sweeper and harass him off the point, look to gank. For singed you can just harass and force him to run off somewhere else.