Lower maximum current rating on available motor driver .
The general solution is to implement PWM on the low side of the full bridges with 75mV to 100mV current sensing to reduce the PWM.  

You can use current input control for PWM or Voltage input.
Current is proportional to torque (Imax=Vcc/DCR).
Voltage is proportional to no-load final velocity but also can limit current due to coil resistance, DCR with Imax=Vdc/DCR

Thus if you compare average current from Rsense with control input, the servo loop gain will regulate the current by adjusting PWM automatically.    

If there is an obstacle, then current sense feedback will protect the drivers while the motor heats up until you stop.  This implies that you need to sense velocity changes too in addition to current ( either visually or with sensors).

These 2 topics (PWM and servo current control) are covered in numerous questions and many websites, so I will stop there.

It is prudent to choose drivers with current drive capacity much greater than you intend to use (>5x) which implies higher efficiency and lower heat losses.

It is also wise to shop for 100W motor servo controllers as they can be better than DIY designs but expect to use a small heatsink and fan or at least give you some implementation ideas.