Sometimes I get routed down dirt roads.

Hey joco, Depending on your unit, you should have a setting somewhere (likely under "Routing" or "Navigation") that will tell your unit to avoid tolls, ferries, u-turns AND unpaved roads. It may take you awhile to dig for it, but every one of the Garmin units I have has it in there somewhere. The newer ones like my dezl have "Custom Avoidances" where you can even plug in the roads you don't ever want to drive on. Good luck and happy trails.

Your manual says that
Tools > Settings > Navigation has several options
Route Preference
Avoidances
Transit Types

Although the Avoidances does not give any more details, it does say "select the road types you want to avoid on your routes". I assume this is where you would find unpaved roads

I'll be interested in what others may say about this, but I can not think of a quick way to prevent that same route from coming up again without some continuing action on your part.

When planning a route, you could look at the "turn by turn" and decide that you did not agree and then add a waypoint(s) that would result in what you thought was a better route.

All depends what unit you have. joco1330 didn't say.
My 3790 learns my routes. I live in a cultesac. It used to always take me down the bottom Route. It's learned that i come in from the top end. I also ended up on a dirt road once with my 765t, Un-ticked
un-paved and all was good.

I set my unit to give me the FASTEST route rather than the SHORTEST route. The shortest way is often down dirt roads. Of course it may be a road that you can only go 10mph on and will take you forever and you may not even be able to make it down the road........

Setting the GPS for fastest route and also setting avoidances to not take unpaved roads should easily solve the problem.