First, the preachy bit: Don't. Trying to disable the user's back button is very bad practice, it's their machine not yours.
Second, the bad news: You can't do it, not 100% reliably. For starters it relies on Javascript, which can be switched off on the client side.
Third, a bit of advice: find another strategy. It would be better for your app to handle back button requests more gracefully.
There is a more thorough discussion than I can make
here[
^].
There are circumstance where you might feel this is appropriate (e.g. a requirement that you can't budge, an intranet app where the cure would be worse than the disease) you can take a look at
this article[
^] (which is worth a read anyway). Even so, I'd avoid disabling the back button pretty hard.
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