There are a few ways (completely hypothetical) ways to salvage this situation, by imposing another limitation that allow FTL travel, but prevents the set up that allows causality violation. A (probably incomplete) list:
- Preferred FTL reference frame - The "default" proposition for an FTL jump-type drive is that you move across space in no time (or a very short time) as measured in the reference frame of the drive when you engage it. The fact that those reference frames can be different for two FTL drives moving at relativistic normal space velocities (or for one drive making successive jumps after a relativistic-scale velocity change) is what allows you to turn FTL travel into causality-violating time travel. If instead, all FTL travel takes place in a single, preferred reference frame (for example, the frame in which the cosmic microwave background radiation has no dipole component) regardless of the velocity of the drive when engaged, it is impossible to create those causality-violating paths.
- Virtual particle censorship of wormholes - A similar causality violation can occur with wormhole FTL if you have two wormholes (each with two ends), where the ends are moving at relativistic speeds relative to each other. The limitation that could prevent this is caused by virtual particles in vacuum (which are responsible for things like the Casimir effect). Simplifying greatly, you can "borrow" energy from vacuum to create a virtual particle pair (usually a particle and its antiparticle) as long as the virtual particle lifetime is short enough that the product of the that time and the "borrowed" energy is less than the Plank constant. Since the Plank constant is so small, that means that macroscopically observable virtual particle effects are typically secondary. However, as two wormholes move closer and closer to a condition in which you can loop through both of them and arrive at your start point in zero time, ever greater numbers of virtual particles will make that round trip, which increases the mass moving through the wormholes enough to collapse them before you get to a configuration that allows a zero or negative travel time (and thus leads to a causality violation).