How can I selectively disable paste blockers

Web forms which have this paste blocker feature may implement it declaratively by using the onpaste attribute for a text box or by binding an event through JavaScript or jQuery.

Contrary to what you stated, I was able to get Derek Prior's bookmarklet to work on Chrome (ver 39 on Windows) with pages that had the attributes (type="password" onPaste="return false") on the password text box

You can also try Chris Bailey's bookmarklet which breaks paste blockers implemented through JavaScript.

This is a bit of a hassle but another alternative is to disable JavaScript temporarily for that page which in turn will turn off the JavaScript paste blocker. For Chrome, the keyboard shortcut to open Dev Tools is F12, to reach the Settings panel is F1 & here you'll find the checkbox to Disable JavaScript in the General section.

For pages that use jquery (basically all webpages). You can paste the following javascript url into the address bar

javascript:void($('input').attr('onpaste', ''));

1.--- This may make the problem go away for at least a few sites: COMPLAIN, and let these websites know that they are DESTROYING, not enhancing security for their users. Maybe they just haven't thought it through....

Here's the text I just copied from Chris Bailey's blog that has his bookmarklet. (Hint: Keep this in your bookmarks).
--> I'd suggest you could send PART of the following to any site that needs to be re-educated:

Re-enabling Password Pasting on Annoying Web Forms (v2)

Security is not to be taken lightly so in recent days I’ve become increasingly frustrated by the insistance of some companies to disable the facility to paste passwords into login forms. Rather then increase security, this cripples those of us using password managers such as KeePass, [LastPass], or 1Password, as the nice long randomly-generated passwords cannot be simply pasted into the password field. Instead users are forced to manually type in passwords which will promote the use of shorter passwords (and thereby weaken security).

I've found this occurring on companies such as Apple, Vodafone and Nestlé.

Thankfully I've found a solution in the form of a bookmarklet. The original idea came from the blog posting Re-enabling Password Pasting on Annoying Web Forms by Derek Prior. Unfortunately his method simply removes the onpaste attribute directly but this doesn't work if the web site is using an event handling framework such as jQuery. I've taken Derek’s original code and modified it to work better with these frameworks.

2.--> Try Chris's bookmarklet available here, but only in Chrome, Opera, or Vivaldi when the page is NOT using JQuery:

3.--> Ask someone to write a comprehensive plugin for all browsers. To get started, here are his comments and source reference from Chris's page:

To use the bookmarklet, drag the following link to your browser’s bookmark bar.

Compatibility Note: The bookmarklet only works in Chrome and Safari due to the difficulty of reading clipboard data in Firefox. It could easily be extended to include IE although I don’t have access to a Windows system to test this. There are other solutions to this problem in the form of chrome extensions (e.g. Paste ITC Password & Allow Paste into fields text although I’ve not tested them) and I would assume similar addons are available for the other browsers. Personally I like the simplicity of the bookmarklet. The code is hosted on pastebin if you want to play with it:
(ED: And his source is also listed below this excerpt, on his linked page.)