Aaaaaaaaaahhhhh I did that when I first started. TOTALLY did that.
Best bet is to order them to a position on mars's orbital path, and wait a year or so for mars to run into it (you will still need to order it to go to mars when it gets close)
just out of curiosity, how fast is the trouble fleet in question?
gnome, try to always make sure your fleets are at least breaking 150km/s else theyll be wasting a lot of time chasing planets.
Once you get your first real engine tech (nuclear thermal), Itll be gone from your problem list forever, as even cargo groups will be averaging well over 500km/s.
Also I advise that any of your scouts and geosurvey/grav survey ships (the ones you use to explore other systems) have over 3000m/s of speed, that way if they run into a black hole, they might survive.