The neatest way I've seen to jam the pin is with a sliver of CDR, carefully filed to shape and superglued to the back of the lens.

It would obviously be a better idea to just buy an M42 body capable of operating the pin! Spotmatics are still relatively cheap and plentiful here, or go earlier with an SV if you're not bothered about a built-in light meter. Either is likely to need a CLA but will be ready for another few decades afterwards.