Waze only allows so many reports per user within a certain timeframe. This is to prevent overusing the feature, point-farming and just annoying others in general. Traffic will be detected by your speed of travel, there is really no need to submit numerous reports. If you sit in traffic long enough, Waze will ask if you're sitting in traffic.
The timeframe for the block to clear is confidential and known only to Waze. This is again part of the feature itself and limits ones ability from spamming the map.

It would be nice if Waze would give users some kind of advice and/or warning before just blocking them, or if it would quietly ignore excessive reports instead.

Having said that, you should also note that (a) your reports have no effect on routing (as karlcr9911 said, routing is based on traffic speeds, not user reports), (b) your reports do result in alerts to other users, but (c) too many alerts in too short a time can be extremely distracting and annoying. Imagine if you are sitting in stop-and-go traffic on 395 and every 30 seconds you hear, "Watch out! Heavy traffic ahead!"