What you describe is an eternal problem for Spanish learners. This string of letters is known as a consonant cluster. These consonant clusters occur very rarely in the Spanish language and never at the beginning of a word. In order to make it easy for them to pronounce, they add a vowel. Consider the name they give to their own country: Espana. Words in their language just don't start with consonant clusters.
To help your students with this, you can invent sentences that they practise. Give them something like the following:
Ask them to repeat it after you until they pronounce it correctly. Then get them to run it all into one word ie "Stephenstrousers". At this point they are producing the consonant cluster in the middle of the word. When they get this correct, you can ask them to repeat it after you with a small pause, so they'll be saying:
Have them repeat it until they get it correct. They can then "say" the word Stephen silently and the imaginary word "strousers" aloud.
You will need to do a lot of work on this. I suggest a few minutes every lesson. Keep a picture of Stephen's trousers pinned up in the classroom. When a student makes a mistake with pronouncing a consonant cluster, point to the picture and ask them to correct themselves.