Set unselectable part of text:

UnselectablePrefixSpanWatcher

import android.text.Selection
import android.text.SpanWatcher
import android.text.Spannable
import kotlin.math.max

class UnselectablePrefixSpanWatcher(private val prefix: String) : SpanWatcher {

    override fun onSpanAdded(text: Spannable, what: Any, start: Int, end: Int) = Unit

    override fun onSpanRemoved(text: Spannable, what: Any, start: Int, end: Int) = Unit

    override fun onSpanChanged(text: Spannable, what: Any, ostart: Int, oend: Int, nstart: Int, nend: Int) {
        if (what == Selection.SELECTION_START) {
            if (nstart < prefix.length) {
                val end = max(prefix.length, Selection.getSelectionEnd(text))
                Selection.setSelection(text, prefix.length, end)
            }
        } else if (what == Selection.SELECTION_END) {
            val start = max(prefix.length, Selection.getSelectionEnd(text))
            val end = max(start, nstart)
            if (end != nstart) {
                Selection.setSelection(text, start, end)
            }
        }
    }
}

EditText.text.setSpan(UnselectablePrefixSpanWatcher(prefix), 0, 0, Spanned.SPAN_INCLUSIVE_EXCLUSIVE)