Blocking Ads by ISP

PPTP can be blocked by ISPs because it (1) runs exclusively on port 1723 and (2) uses non-standard GRE packets which are easily identifiable. See PPTP on Wikipedia.

The solution is using OpenVPN protocol instead of PPTP. Here's a tutorial by BestVPN that covers setting up OpenVPN on a linux VPS.

There are increasing degrees of obfuscation that can make OpenVPN traffic pass under your ISP's radar. Ordered from easiest implementation/detection (they correlate) to most difficult:

1. Default Configuration - Just install open VPN. It runs on TCP Port 80 by default, same as regular web traffic. Many operators block encrypted packets on this port, so this is the probably the first step covered in the ISP's VPN-blocking strategy.
2. Use TCP Port 443 - Running OpenVPN through TCP 443 (same as HTTPS/SSL traffic in the browser) is the next step. This puts your encrypted VPN traffic in the same bucket as the data you exchange with secure sites such as your bank. Certain deep packet inspection methods can filter out VPN traffic on this port, so your ISP might have that covered too.
3. Obfsproxy - Easier than it sounds. It's a a project developed by Tor to allow access to relays being blocked, but works for VPN traffic as well. Needs to be installed on both VPN server and client machine.
4. Via SSL Tunnel - Now the bigger guns. Using stunnel one can send one's VPN traffic through a second layer of SSL encryption, effectively masking VPN traffic as standard SSL traffic. This method is imperceptible to known deep packet inspection methods.
5. Via SSH Tunnel - Stealth Bomber. Even in highly censored environments, SSH will almost always go through, as it is a core functionality of modern server architecture and the censorship tactics themselves involve SSH.

These methods are covered in another tutorial by BestVPN here.

You can start with a plain vanilla OpenVPN setup. If it works for your ISP, all done. Otherwise, try each step in order, checking back to see if you can get through after each configuration change. Note that SSL/SSH tunneling methods are more secure than Obfsproxy, but do cause a performance hit from the double encryption.

There are three ways to beat this:

1. Only go to the https: version of the website. There are browser add-ons that help you do this (e.g. HTTPS EveryWhere). This won't help you if the sites only have a http: version.

2. Use a Virtual Private Network. There are many VPN providers available, probably some free ones as well.

3. Install an Ad blocker (like AdBlock Plus). Chances are that the IP adresses that the adverts come from are already in the block list of the add-on. If they are not, you have to trace the domain names or IP ranges that they come from (they may very well come from one ad provider), and manually add that information to the block list. Data from those web addresses will not be retrieved.

Methods 1 and 2 ensure that your traffic is encrypted when it passes through the ISPs' computers. They cannot see (and change) the HTML/Javascript that way.^*

Although you say I can't change my ISP, that would be the preferred solution. Leave and tell them you left for that reason. Losing customers is the only argument they will listen to, and this will help other customers too.

If you are absolutely sure that you ISP is doing this, I invite you to edit your question and name them.

^{* This assumes that your provider has not asked you to install their certificate so that they can decrypt your traffic. This is unlikely, I have not heard from providers that do so. If they do, method 1 is not going to work. A VPN still will.}