Most people getting on stage are doing it for a laugh, don`t have any intentions of breaking anything, it`s their night out and they`re just getting a bit merry and over-excited. I do find this happens more with covers bands in regular pubs rather than originals bands, no idea why.
But if their behaviour is getting to a point where it`s unsafe, either for them or the band then simply give them a good firm push and yell f**k off at them, and be prepared to hit them full pelt if needed (tho not if it`s ladies, irrespective of their behaviour I`m sorry, it might not be equal treatment but I`m not walloping women).