Bricks are a bad idea
a brick in a box would be entirely the wrong shape, for posting without paying the right postage..
Might I suggest that you take the offending pamphlet, and write a short not asking not to be mailed again.
Then, and I must stress that this is so that neither the pamphlet nor your note get damaged and thus become illegible during its passage through the postal system, and for no other reason.
Stick the letter and pamphlet between two offcuts of plywood. You should pick your pieces such that they will protect you note, but they should not be so thick that the final package will not fit through a letter box. Wrap or put into a sturdy envelope and address (remember to write clearly and use the postcode - you don't want the Royal Mail to send it to the wrong person).
Now take it to be posted. Since the package fits through a letter box, if the post office should happen to be closed then just put it into a pillar box. You did put a stamp on it didn't you? No? Oh dear. Well you can't interfere with the mail to retrieve it, so I guess the recipient will have pay (including underpayment surcharge) if they want to receive it........
[this is of course entirely hypothetical and should not be used as guidance for a course of action]