One possible explanation for OP's difficulties is playing multiple forts in the same world.
Once one fort has become infested with werebeasts, subsequent forts may be more likely to see those same infected dwarves appear as migrants or visitors.
I'd definitely like to see more interesting behaviour from werecursed civilization members. The first transformation should be a surprise to them, but after that they should become aware of their curse and act accordingly. Some might flee into the wilderness to live alone, only attacking settlements when they transform. Others might try to live normal lives but take precautions every month to lock themselves in a room before they transform. What definitely shouldn't happen, but seems to be the norm at the moment, is for them to live completely normal lives unaware of and unaffected by their curse until they enter a player fort and become a ticking time-bomb of death and destruction.
If they've been living in the world for at least a month then there should be some explanation for how they've coped with their transformation. Maybe they've been chased into the wilderness by town guards already and are just posing as a migrant to sneak into your fortress, but in that case it should be possible to uncover the truth via interrogation.
On a practical level as a player your only real defence is to treat all migrants and visitors as potentially infected and quarantine them during the full moon. The ones that don't transform can be safely integrated into your population while the infected ones can be disposed of however you see fit. Dwarf Therapist lets you sort dwarves by migrant wave which is very useful for this sort of thing, and if you want to cheat it can also be configured to highlight cursed dwarves.