the first paragraph pretty much sums it, but as another user posted, they can just use incognito windows to look at lairs without penalty, so it's a pretty easy workaround. since forums aren't limited by blocks, they can just post.

you could in theory report for block evading if you've blocked them, but your mileage may not get far depending on how staff handles it. according to their Stalking section of the Harassment page on their FAQ, they include "attempting to get around blocks using third-parties" under their definition of stalking.