who are the modern day royals of the united kingdom who reverted back to roman cathol

as the head of the church of England, what did the queen do to them?
excommunicate? strip of title?

princess Micheal

The cousin of Queen Elizabeth, the 73 year old Prince Edward, Duke of Kent was only age 16 when she became Queen. He has been doing royal duties since the age of 18. In 1994, his wife converted to Catholicism after 33 years of marriage, but the Duke was not removed from the line since she was not Catholic when he married her.
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Their oldest 46 year old son, George Windsor, Earl of St Andrews and two of his children (20 year old Edward Windsor, Lord Downpatrick) and (16 year old The Lady Marina Windsor) have since converted. They have been removed from the line of succession. The last child is is age 13 (The Lady Amelia Windsor) and is expected to convert when she gets older. Right now she is still in the line (currently ranked #25).
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Their other son is 38 year old Lord Nicholas Windsor. He actually got married in the Vatican. His 1 year old son, Albert Windsor , was baptized in the Catholic church and has been removed from the line of succession.
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The Queen's youngest cousin, Prince Michael, married a Catholic woman 30 years ago. He is out of the line of succession, but his two children were raised Anglican so they are still in the line (ranked #31 and #32).
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These 6 people (7 when Lady Amelia converts) are the only ones in the Windsor family who are excluded. There are obviously more distant lines that are totally Catholic. The sisters of the King of Norway married Brazilian men and are Catholic.
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Prince Michael was born 7Th in line to the throne (and for a short time was ranked 6Th). By the time he got married though he was 16Th. Prince Michael is still the highest ranking royal to be excluded [at the time] because he was Catholic. In present day the son of the Duke of Kent is the highest ranked person to be excluded. His mother, the Duchess of Kent, is not in the line.

EDIT: I think they will change the law in a heartbeat if Prince William falls in love with a Catholic girl. The last of the laws discriminating against Catholics in common life was changed in 1828. Changing the law would require that the monarch no longer be head of the Anglican church.

The Duchess of Kent is the highest ranked member of the Royal Family to convert to Catholicism.

Princess Michael was never an Anglican she has always been a Roman Catholic, upon their marriage her husband was removed from the line of Succession, however, the Duchess of Kent outranks Princess Michael.

When she converted, the Queen approved. The Duke of Kent (and his children before they converted) are so far down the line of succession that the chance of them succeeding to the throne are nearly non-existent.

they are not Catholicism.

When Prince Michael of Kent married the Catholic Marie-Christine,he had to step out of the Line of Succession.Nothing happened to The Duchess of Kent,when she converted because as a spouse,she wasn't in Line of Succession.The Acts of Settlement requires the monarch to be Church of England,so whomever is in Line of Succession must be Anglican/Protestant.This requirement will probably be dropped one day.

Others have given the names. You also asked "What did the Queen do to them? Excommunicate? Strip of title?".

No. The Queen can't excommunicate them - they effectively excommunicate themselves from the Church of England by leaving it. She could strip them of their titles but should wouldn't want to. They excluded from the succession, but that's not up to the Queen: the Queen has no control over who succeeds to the throne. It's a law passed hundreds of years ago that bans Catholics. Unfortunately in order to change it, you'd now have to get every country of which she is Queen (there are 16 of them) to agree to do it.