The difficulty is adjusted every 2016 blocks by the whole network so as to average the Bitcoin generation to a block every 10 minutes. If the coins were on average generated too fast since the last adjustment, the difficulty is increased, if they were generated too slow - it is decreased.
What the rich can only do is do their best to have the highest possible hashing speed. This would make them brake the block more often (they would earn more money), as well as increase the difficulty higher, so people with lower-end machines could not brake a block without using a pool in a reasonable time (as is the case currently).
What the rich can also do is hoard the Bitcoins, so their value would increase. Keep in mind that even if someone would own 99+% of all the Bitcoins the currency would still be as usable as with bigger pool of coins (Bitcoins currently can be divided to up to 8 decimal places, and there is nothing stopping a future implementation from dividing them up even further).
Of course if someone wants to depreciate the value he or she can undermine the trust people have in the coins. For example, the hacking of Mt.Gox caused a lot of people to be more reserved when handling the coins.
On the other hand, if one wants to increase the value of Bitcoins, one should promote them, convince new people to invest in them and be a contributing member of the community.