How do you make a coin resistant to soft forking?
You can't prevent miners from enforcing more rules than the rest of the network is. However, to actually turn that into a soft fork, a significant portion of the network also needs to start enforcing it.

However, the only way a softfork can affect users of the system in a negative way is by disallowing certain transactions that used to be useful. That problem is nothing more than the inherent censorship rights a majority of miners have over the system. The solution to that problem is not trying to prevent enforcement of rules (which is not possible; miners can do it even without announcing it), but by improving the privacy of the system. If miners cannot see what transactions are doing, they cannot selectively block.