Maybe there is somewhere the ‘Kellys’ could park that would not obstruct access to facilities? If so perhaps they could be granted exclusive use of such a space in exchange for some benefit to the OC generally (eg. a fee).
I like the bollard idea, but why should the rest of the OC pay for that? A notice to comply could include the suggestion that if the notice is not complied with a bollard would be installed and orders sought to recover the cost for its installation. If the OC gets compliance from the bollard threat, that’s good. If the OC has to put in a bollard, the cost is small anyway. If the OC gets the cost of the bollard back, that is all the more satisfying.