The גמרא asks if a נזיר was warned not to accept upon himself נזירות while in a בית הקברות and did it anyway, do we say he doesn’t get מלקות unless he hangs out there a bit (כדי השתחויה) or not? תוספות in ד"ה אילימא asks why we don’t call this a לאו שאין בו מעשה since he didn’t do any action from the time he was warned? תוספות answers that the נזיר not leaving is considered a מעשה. What exactly does that mean? It is not so clear and there are several possibilities. תוספות in שבועות on דף י"ז ע"א ד"ה או אין צריך as well as the משנה למלך in הלכות ביאת מקדש פּרק ג׳ הל כ"אunderstand that since at the beginning there was a מעשה when he came in, that מעשה can continue on forever. This is similar to the case in our גמרא where a נזיר is brought into a בית הקברות in a שידה תיבה או מגדל and then someone removes the plaster on the bottom where we consider it a מעשה to get מלקות even though the person himself didn’t do anything. You see from there that as long as the process started with a מעשה, that’s enough for it to be considered a מעשה even later on. It is also similar to the case in מכות דף כ"א where someone is told to take off כלאים where they are considered doing a מעשה by simply not taking it off. The issue with this is תוספות by us really doesn’t sound like he meant it's considered an action because of what happened originally, which is what the קהילת יעקב points out in נזיר סימן ה. Rather, תוספות sounds like he means that simply not leaving when you could leave is called a מעשה. This is also what the ריטב"א in מכות on דף כ"א sounds like and is certainly the way it is quoted by the כסף משנה in הלכות כלאים פּרק י׳ הלכה ל. The only issue with this פּשט is that it's hard to understand. How does not moving when you could move count as doing an action? That is why the שאגת אריה in סימן י"ב rejects this possibility.

Another possibility to answer תוספות’s question in general is a מגיד משנה brought by הגהות מהר"ב רנשבורג. The מגיד משנה says that whenever a מצוה could be done with a מעשה, even if you do it without a מעשה, it's still considered a לאו שיש בו מעשה and the person would get מלקות. The only time you don’t get מלקות is if you couldn’t have done the איסור with a מעשה (e.g. לשון הרע). This is how the מגיד משנה explains why the גמרא says that if someone yells at an animal and that causes it to stop eating, he violates the לאו of לא תחסום שור בדישו and gets מלקות. In that case he did not do an action other than talking and he is still considered having done a מעשה, so you see that the fact that the person could have done it with a real מעשה is enough to call it a לאו שיש בו מעשה. Interestingly, the חינוך in many places says the exact opposite—if an איסור was done with an action, but it could have been done without an action, it is considered a לאו שאין בו מעשה.