Decaf coffee (or tea) will still have more than zero caffeine. Allowable amounts vary between US and EU, but multiple repetitions of the process are needed to achieve either 97% or 99.9% caffeine free respectively.
A single pass through a decaffeination process can result in leaving up to 20% of the original caffeine content.
Surround the park with shipping containers, and done! It is now 100% dystopia appropriate housing!