Solving the halting problem is neither necessary nor sufficient to prevent DoS attacks.

It isn't necessary, because settings timeouts or other resource restrictions works way better to prevent DoS.

It isn't sufficient, because even if you can prove that a program will halt at some point, this alone doesn't tell you how long it will take. What good does it do to know that the program will run for 10 years before it halts? By that time, service will already have been denied. Even turning hash table lookups from O(1) to O(n) (still very much terminating!) can result in a DoS.