For anyone who has the misfortune of crossing paths with the wayward Dementors that the Ministry seems incapable of keeping under their firm control, there are extra cheering charms and an abundant supply of chocolate and chocolate treats on hand at Mungo's. Simply stop by and the Welcome Witch will direct you to the proper floor.
Additionally, taking on a Dementor head on, no matter who you are, is not a good idea and the consequences of doing such a thing could be lasting and dire. If you see a Dementor, the best option would be for you to leave as quickly as you can without drawing any attention to yourself. In this case, do not try and be a hero.
{Selwyn's.}This warning is double for you. I do not want to see any of you land yourself in Mungo's because you thought you could take on a dementor. Do NOT try and do such a thing.