It's a game table emulation, not a game table
As the DM, you need to both recognize and accept that it's a different game/gaming experience when played in the Roll20 (or similar) venue.
- The DM and the players lose the synergy and intimacy of the table top experience and the in person experience. (From a personal experiential level, this is what I miss the most). Where visual cues and subtle body language pass information at the table, this capacity is reduced in Roll20 even if you use the voice and video features. (Talking over each other is actually less chaotic in person than it is over the VOIP medium).
You gain the ability to play, as we do, across a massive geographical
distance and to use some neat automated tools (I have come to love
the Roll20 die roll macros and our DM's ability to use the lighting
and shading features).
What the text / "whisper" feature replaces is part of what you have
lost by not being at the same table in the same town. Yes, it can be
abused. Part of that is the problem of pacing, since if things slow
down anyone with even modest keyboard skills can keep themselves
entertained using whisper without being disruptive.
- On the one hand, that means that various players cracking jokes and asides won't interrupt whomever has the spotlight, on the other hand the meta/information foul is easy to commit.
- While "breaking immersion" may be complained about here, the
immersion when playing over that medium is different, at best, then
in person, and is easily lost for a variety of reasons. (Spouses among the top causes in our case).
First step: get expectations to match
As a DM, the practical limit of what you can do is express what your preferred limits are for meta gaming up front. You either get some level of buy-in from your players, or none. If the initial result is "none," it is necessary to expend the effort to get on the same page regarding meta gaming. Unless you come to an equitable balance of expectations, as a group, starting the game will be for you an exercise in frustration.
We are in this to have fun together
Once the expectations match, ask the players to help you enforce it. This means that you expect the players to nudge/mention that "too meta" has occurred to whomever is doing more meta than the group agreed on. Don't be stuck in the position that you are the only one who has to be alert for and catch it. (Heck, you are running the game, that's work enough!)
Embrace the challenge
If you try to micromanage it over the fiber-optic network, you'll fail. If you get the players to help you encourage or enforce the limit if metagaming, you'll have greater success. In that respect, getting the group to buy in, it's a lot like the TT environment.
It's what our DM does and it works for us. (Our players are in: Texas, Virginia, Chicago, Illinois, Michigan(UP), California, Washington(State), New York).