Sorry to say it's all protected free speech. The only way I know to cut down on the mail you receive is to contact both political parties and ask to be taken off the mailing list. Tell them you know who you are voting for and they aren't going to change your mind. They will most likely ask who and it won't hurt to be honest. They don't want to waste money on decided voters. This won't help with unaffiliated groups that are working independently of the major parties. The other strategy is to vote absentee as early as possible. That won't affect mail but might cut down on robocalls. Although those seem to be less of an issue this year.