How do manufacturers limit bandwidth in oscilloscopes?
You should realize that sample rate (1 GS/s) and Bandwidth (70 MHz) are different things!!!

They are related in that a certain sample rate dictates the maximum bandwidth of the signals which can be sampled accurately. This is set by the Nyquist frequency

The Bandwidth of the oscilloscope is most often limited in the frontend of the oscilloscope. The frontend is the input amplifier including protection circuits and range switching (which changes the voltage gain of the frontend). There might also be an anti-aliasing filter (a low-pass filter) present.

On way to make the frontend's Bandwidth changeable by software is by simply switching on/off a capacitor. This is done in the Rigol DS1054Z as shown by Dave from the EEVBlog in this video. That capacitor can simply be part of an RC Lowpass filter (the anti aliasing filter!) which sets the Bandwidth.

It is theoretically possible to (also) limit the sample rate and/or do post-processing to limit the bandwidth but this can result in aliasing effects and requires processing power. Switching a capacitor is much, much simpler. Also, that would limit the oscilloscope's bandwidth in the same way as it always has in analog oscilloscopes. You can view a 100 MHz signal on a 70 MHz oscilloscope but the 100 MHz will be attenuated. So you might measure 1 Vpp while the signal is really 1.5 Vpp for example.