Can radar/lidar sensors for self-driving cars be designed to ignore deliberate interference?
You'd be surprised.

This is actually topic of ongoing research, several PhD dissertations and a lot of money invested in the auto radar jamming industry. Military researches were made for jamming/misleading radars  and also to prevent that from an opponent. 

The efficacy of jammers made them illegal in most countries in the world, not that many drivers were caught and fined for using it since it's hard to find which one used the jammer from many cars passing by.  

Just search google for police radar and laser jammers and you will find plenty of options. 

There is a major difference that makes the requirements for an auto radar much higher to be used for unattended driving, they must to be available any moment.

Successful jamming or misleading for just one second may in this case lead to a major crash unlike in police radar where you must blind the radar for a long time and the worst that can happen is that you can pass without a fine( which is bad but not that bad)

Even military applications can survive few seconds of blindness without major damages. 

Lidar is somehow more resilient to jamming but it cannot be used in close proximity as crowded crossroads.

What can be done? 

Somehow, the best is to combine data from all sensors and rely less on weak ones. Tesla's bet is to use visual camera as the main sensor in taking decisions which makes sense to me since the best self driving system ever use only two cameras and two accelerometer/gyroscope units. That is a human driver. I pointed this in my answer to the other related question but it seems that there are many fanatics ready to blindly rely on the new advanced technologies. It seems that people that used radioactive blankets for treating arthritis (as the cutting edge technology of their time) in the early 1900s still were capable of having descendants. 

The industry is still playing with different solutions that might work while based on the good will of the others but in the end I think that the authorities will transition to smart roads equipped with sensors and AI that will guide the vehicles which overall is much cheaper and much efficient. Of course this will take "self driving" out of it's meaning.