At the moment, the task is insurmountable. If we live beyond the moment, we may see time as a partner in the survival of all mankind.

Every planet in the solar system holds a series of lagrange points (Pluto is likely too small to have much more than L1 and L2 as feasible navigable traps) which could handle the orbit of a small fleet of sleeper satellites, each watchful, each networking current data to other sleepers around the solar system, each fully fueled and ready to pounce.

Interstellar visitors would likely be traveling at much higher speed that our current bevvy of comets and asteroids (100,000 mph), simply to hold and maintain a stellar escape velocity, even faster than our own interstellar probes. They would come as a flash across the system.