If it happens repeatedly, declare the passage of all Russian ships (or possibly starting with ships of the type involved in the incident, allowing other shipping and giving Russia a chance to stop abusing it) "prejudicial to the peace, good order or security of the coastal State" and deny passage through territorial waters. Extend the territorial waters between Finland and Estonia to the full 12 miles without the current corridor in between.
Russia understands and responds to strength better than to diplomacy and appeasement.
The ships involved aren't warships. They're ostensibly civilian vessels. Also other people mention that accidental fiber cuts happen all the time. Are we going to drone strike Russian civilian ships on the off chance is malicious?
>Russia understands and responds to strength better than to diplomacy and appeasement.
The best way to stop someone committing war crimes is... to commit war crimes ourselves?
There seems to be consensus that this was not an accident (politicians have stated as such), and treating it accordingly would show Russia "no, you can't just pretend it was an accident and expect us to do nothing".
On the off chance it actually is an accident at some point - that's the downside (for Russia) of having pretend-accidents too many times.
The alternative is ignoring it "because we can't be sure" until we get to ignore the "little green men" that totally aren't Russian when they come across the border...