Official website of ui select2
[Edit 2018-11-12] You could always do a "display: none" on the x span:
span.select2-selection__choice__remove {
display: none;
}
You could also use that to disable click on it with pure JS.
And there is also a "locked" option: http://select2.github.io/select2/#locked-selections You can then control, per data, which one can't be removed from the selection.