How do you make a coin resistant to soft forking? Soft forks are probably worse than hard forks, since it is hard for a client to reject a soft fork (indeed, that is the point of soft forks). The only way to do so is either to gain 51% of the hash power, or to do a hard fork rejecting it.

How can you make an altcoin resilient to soft forking (or only allow certain types of soft forks)? I think doing this is in general might be impossible, but making it harder would be good. Having no soft fork policy built into the coins philosophy would help, but probably isn't good enough.
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.