bad installer

we called out a dstv installer DSTV SANDTON 81 Woodmead Drive to help fix a problem with our decoder that was rebooting all the time. they come through fixed 1 problem but left with another, 1 of our decoder's is still rebooting all the time. they promised to come back to sort this out but have not done so and we can not get hold of the as they do not answer there phones. we paid them already , we would like to get the money back and get someone else. what can we do?