Did the Spanish Conquistadores try to stop the spread of disease to the indigenous populations of the New World, or did they use it as a way to conquer the natives?
There is no evidence that either of these things happened. In fact, it is hard to imagine that either of them could have happened.
In the early 1500s, Europeans had no good idea as to how diseases spread. This is, for example, why they had such problems with the bubonic plague. They thought that diseases were caused by things like bad air. With no knowledge of microorganisms, they could not hope to understand the process by which diseases spread.
With no knowledge of how diseases spread, they could not try in any effective way to spread the disease or to contain its spread. They simply would not have known what to do in either case. It is possible that individual Spaniards did do things to try to stop the spread of disease (or to increase it). However, neither of these was an official policy. Even if there had been official policies, they would not have been likely to work because of the lack of medical and scientific knowledge at the time.