Advice

From my experience, fantasies start from two things: triggers and random thoughts.
To deal with triggers: you need to minimize (or attempt to stop completely, which is better) seeing things that you know may arouse you - movies, TV shows, random videos with inappropriate scenes. When walking on the street, be careful where you look (if you have glasses, and you're in a familiar area, take them off). Be careful what reading material you have, etc.
To deal with random thoughts: I use the 5 seconds rule - when a negative (PMO-related) thought enters my mind, I have up to 5 seconds to think of something else. With time, I've been able to lower that to 3-2 seconds.

If you deal with those two issues, I think you should be able to get past sinking into long fantasies.

 

What has helped me is simply saying 'No'.

When such thoughts come to mind, try to tell yourself, "No. I don't do that anymore".

And then go find a task to do. Try not to be by yourself or isolated.

 

Personally, I don't feel we can control these. What we can do is learn to (1) acknowledge their existence and (2) respond to them differently (ie: in ways that do not lead to addictive acting out). In many ways, these two describe the core work of recovery.