https://pe.usps.com/text/dmm300/505.htm
> 1.3.8 Improper Use of Labels and Misuse of BRM Cards and Envelopes
> Improper use of BRM labels or misuse of BRM cards or envelopes should be handled as follows:
> When a BRM label is improperly used or a BRM card or envelope is misused as a label to return an unsealed item such as a brick, two-by-four, or similar item, the Postal Service may treat the item as waste to be disposed of at the discretion of the Post Office.
> When a BRM card or envelope is misused and affixed to a sealed item, the permit holder will be responsible for payment of the applicable Retail postage and per piece fee.
I think this is saying, "Attach it to a box with a brick in it." But neither do I think your local postal worker would be terribly amused.
Just send them their crap back with a "Please remove me from your list!" scrawled across the response. It does actually work, eventually...