If you want to get rid of the radiant Minutemen quests altogether, there's a workaround. Send Preston to a settlement that you rarely visit. Don't listen to Freedom Radio, because you get the radiant quests through there. If you use the Castle a lot, turn off the speakers for Freedom Radio (with the blue breaker box at the base of the tower). If you have some of the radiant quests in your quest log, just leave them there and eventually you'll fail them. This removes it from the quest log (with no serious consequences aside from a slightly reduced happiness rating). You'll get a notification to go talk to Preston, but it won't trigger a new quest. Just stay away from the settlement that he's at, and he won't be able to talk to you to give you a new one.
Occasionally you'll get these radiant quests from the settlers themselves. Usually if you see a settler standing in the same spot for a long time, they're waiting to give you a radiant quest. Just avoid these settlers. Give them a wide berth when walking through. If you get stuck and one starts asking "Did the Minutemen send you?" don't respond, and just walk away. It won't trigger the quest.
This is what I did. I kept getting the same ones over and over again. I did 'Kidnapping at Abernathy Farm' three times. Each time a settler was being held hostage by the same group of raiders at the Federal Rations Stockpile. It was getting old. Once I get through more of the main game I'll probably take on some more of these radiant quests, but for the time being it's great to not constantly have these quests to do.