Altium : Rule on connector
Almost all PCB layout programs assume that the board will be produced commercially, with plated-through holes, so the programs don't make any provision for single-layer designs.

However, you could place a solid fill on the component side copper, not connected to any net, around the connector (or perhaps over the whole board).  This would force the program to connect to the connector on the bottom side.

For a simple board, you may find it best to route it manually, then you have full control of where the connections are made.  Even with (perhaps especially with!) complex boards, I do a lot of manual routing.