Usually there would be a bylaw about a lot owner not doing anything that unreasonably interferes with the reasonable use and enjoyment of the common property (that is ACT wording but probably similar elsewhere). That can cover a lot of things. It could be argued that storage of unsightly mess in a lot owners parking space is interfering with my reasonable expectation that I could use the common property (in this case the shared part of the parking area) without being visually assaulted by ugly mess.
I would use this and any more specific bylaw and concern for fire risk and anything else. List them all. Ask people nicely first, then more firmly, then go to the tribunal. Be utterly fair. Give the same letter to all offenders on the same terms.