First, it depends on whether trial in absentia is legal in the prosecuting jurisdiction. In the US, trial in complete absentia is prohibited by the due process clauses of the 5th, 6th, and 14th Amendments. Italy on the other hand does allow such trials. Second, even if a person is convicted in absentia, that does not mean that the perpetrator is stopped in any sense. This being about law, I will not discuss the option of illegal enforcement (even when legal under the laws of the prosecuting country), however this is an obvious way to overcome the lack of an extradition treaty. In general, if a nation will not extradite a person to another for prosecution, they also will not enforce a foreign judgment against the person. An extradition treaty may not be necessary: if it is legal for country B to expel undesirable aliens (or citizens), that could open an avenue for legal arrest (in country C).