B >     The most common wasp killers are effective for about 24 hours.  (...)
MM> Yea, we've found that too - new wasps seem to move in a few days
MM> later.  (...)
MM> Even when we destroy the nests, they come back and start building new
MM> ones  - any long term solution that you know of? (...)
    Well, under the eves of the house, we caulked all the cracks that they 
were crawling into and scraped the old exterior nests off the facia boards.  
I don't supposed that'd work too well on your century plant.
    Short of totally removing whatever the attractant is, in this case the 
entire plant, I'm not sure what a good solution is.  Any chemical powerful 
enough to have a long term effect is probably neither something you want in 
your yard nor something that would be too good for the plant.  Unfortunately, 
I just don't know enough about the wasps themselves or what it is about the 
plant that causes them to prefer it as a nesting site.  My best guess is that 
the tight spaces between the leaves makes just the sort of home, like the 
tight spaces in the cracks between the boards on the eves of our house, that 
wasps prefer.
        Byrd Mann
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