This experience took place back in 2014 I believe, when I lived in Portugal with my ex-boyfriend.
It was night-time and me and my ex were watching a thriller/paranormal movie. The volume was pretty high, to make ourselves more scared.
I think we were past half way through the the movie when we suddenly hear something through the loud sounds from the TV. What we heard was "ahhhhhhhhhh". It was not a scream, it was more like when you got to the doctor and he wants to check your throat and you have to go "ahhhh". And it was coming from a woman.
We knew right there and then it was not coming from the movie, because the scene it was showing didn't have any females in it, and it was coming from outside the bedroom.
My ex turned down the volume, so did the voice. The voice was actually lowering at the same speed as the sound of the movie. (Once again, the voice is not coming from the movie)
We get off the bed and open the bedroom door. That's when we hear the family dog...she's growling. She only growls when she feels threatened or if something is very wrong. We even went outside, but nothing, it was completely quiet. That's when my ex suddenly says that the voice came from the second floor. We looked at the dark staircase leading up to the second floor.
We knew nobody was up there, cause we were all alone in the house.
The family dog didn't stop growling for maybe ten minutes.
When we went to bed later on, I could have sworn that I had heard a scratching noise from outside... On our window.
I am not sure that a spirit would attach itself to something that likely did not belong to it in its life (like this flower which most likely was placed on the grave afterward). Its possible, but I don't have much experience with that.
However, I do think it is significant that you (and your ex) believed that it may have something to do with the spirit. And by tossing it back into the graveyard, you in essence did some sort of ritual to tell the spirit to move on (or stop bothering you). It seems you did not do a formal ritual, for example with burning sage or holy water. But perhaps the fact that you assigned some power to the flower (assigning it as the cause of the disturbances) and then made your intentions clear about the spirit not bothering you (by throwing it back) were enough to stop the occurrences.
When I read other people's postings of rituals to rid themselves of unwanted spirits, I read many different varieties. But the core of each of these is expressing your intention to the ghost that you don't want it around anymore. Maybe your simple act of assigning power to the flower and returning it was enough of a "ritual" to bring this to conclusion.
In any case, it is very good that you are not bothered by it anymore.
James Bond