Get sick from some local bug, drop off a bug that mutates in the local wildlife and kills off a keystone species, drop off an invasive species that ruins a local ecosystem, start a forest fire that causes animal stampedes, similar stuff?
It could be fun, for interdicting trade how much would the Capellan colony be willing to spend to block that trade? Or would it be better if the Capellans tried to get the existing colony under their control, politicking, underground manipulation, public opinion, etc.
Eventually the group trying to keep the Capellans out has to track them back to their base, and deal with them at their source (Mech fight!)