So this small ruleset in the Apache reverse proxy config on my Mastodon instance seems to work fine for the time being:

RewriteCond %{HTTP_USER_AGENT} \(GabSocial\/ [NC]
RewriteRule .* - [F]

[10/Jul/2019:16:46:32 +0200] "POST /inbox HTTP/1.1" 403 4105 "-" "http.rb/3.3.0 (GabSocial/2.8.4; +https://gab. com/)"

Unfortunately providing an error code other than 403 seems not as easy.

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