Can't Block Number

Hellow Gals and Guys - 

I have a prepaid T-Mobile account that I've had for some time.  I use it with a Lumia 640 Windows Phone.  Someone who I don't want to have my number got it and I am now receiving regular texts from them.  I tried to block it, but the texts still come through.  It says "Media Content In this Message" when displayed n the text list.  When I hit the number it says "Get Media Content Now" which I can select and it downloads the message.  I don't wish to receive these texts, but is there some way to block them?  Also this is the number I give out when an institution or someone I don't know well asks for my number, so I need to have access to it, but I only put $10ish a year on it and this person will go through that soon.  How do I stop this?  Thanks for any help.