We need just a simple script to delete messages by regular expressions.
Also it should be after some random or desired time to protect board owners from observers (parsing endchan and searching big data on network providers in Russia by police) to make it difficult to distinguish when board owner is actually online or this is just a script.
Just it.
Copy same code from filters and modify it to delete posts at some time, but check time globally or when the board is accessed.

Just add this fields in board settings:
 
Anti-spam
Enable: yes no
Random time: yes no

Time to delete spam
Spam delete times: 1-4 (how much times spam should be deleted)

First time: (time of the day, if not random then every day at this time)
Second time:
Third time:
Fourth time:

Random time
Time span for random time: (minutes, add large span about from minutes to few hours to desired time)
Minimal times: 1-4 (if random enabled spam will be deleted few times: from minimal (1) to "spam delete times" (1-4))

Rules to delete posts
Rule 1: text field for regular expression
Rule 2: text field for regular expression
Rule 3: text field for regular expression
etc

And delete spam when time is come, check just last few days posts to limit cpu usage. 
This should be easy for your server and very useful for people there.
We need modern things today to make or place safe and comfortable.
Fuck kremlin bots.
Please do it dude.