No, that's not acceptable. They are using your resources to fatten their stock. There's no helping the fact that animals stray sometimes but the neighbourly thing to do is retrieve your stock ASAP and repair your fences. Starying animals can spread worms and ticks.
You should contact your local council and ask what their regulations are regarding straying stock. They'll probably impound them for you and if the owners don't claim them by the 10 day period you can buy them and sell them at the special sale *evil giggle*.
If you're not as mean as me you could warn them that the council will impound them if they're not permanently removed within a couple of days.