I also do my best to report spam and I don't recall any spam to remain on the forums for more than 6-12 hours, so I think it's not a problem on the forum at the moment (mods seem to do their jobs, also, if they won't be able to handle it, they can always recruit more mods).
Though I like the suggestion, but as @neonmateo pointed out, it's not really okay to restrict new players from for example seeking help. Imagine they have an installation or unlocking issue: they won't be able to ask for help here (on the official forums!), and since they can't go online to drive the required distance, in the end they can't get help. There might be a way around this: make only one forum visible (e.g. Technical Assistance), and when they have 5-10kms on the record, open up the whole forum. Note, this might concentrate all spam into the technical assistance forum - that's not good either.
I still don't think the spamming issue is that huge right now.