How can I switch solar panels on and off with a microcontroller?
What you are asking for would ideally be handled by a properly implemented charger and balancing BMS.

However, a somewhat informal but entirely doable way of shutting down solar input is to use a suitable rated MOSFET to short circuit the solar array - or a portion of it. The MOSFET MUST be either fully on or fully off.
The solar array is not harmed by short circuiting and dissipates minimal energy. 

Power dissipation = I^2R. At 8A and say a 10 milliOhm MOSFET power in the FET
= 64 x 0.010 = 0.64 Watt.
10 milliohm Rdson MOSFETS are common and cheap and even lower or much lower Rdson devices are readily available. At that power dissipation heatsinking is trivially easy. 

A relay could be used for the same purpose. However, opening a 100V 8A DC contact requires switches designed for the task. At those power levels the system is potentially lethal - both due to the voltage alone and also due to the arc potential.

Unless there was a good reason to run the panels in series at 100V+ I'd favour operating them in parallel at 35V.  

Related:

Using 3 x 35V panels in series to charge a 29.4V max 7S battery pack sounds very strange UNLESS you have an MPPT controller.  IF you have an MPPT controller that's (potentially) fine.

A BMS usually both detects cell overvoltage AND drains current from the charging source to prevent cell overcharging. As long as the battery is reasonably well balanced a balancing BMS will be able to handle the current needed to maintain balance.