I agree with Peter. There are more appropriate ways to politely decline an invitation, and the phrase "I am forced..." makes me wonder who or what is forcing you. Usually, when we decline an offer, we start with a phrase such as: Unfortunately, ... Regrettably, ... I am afraid that ... I am sorry, but ... Then we give the reason for declining the invitation. Saying "no" is far too direct and blunt. It sounds as if you dislike being invited, and that you don't care what the other person thinks. If you want to be absolutely clear, then you can add one of these phrases at the end: ...therefore I cannot accept your kind invitation. ...therefore I must decline your kind invitation. This is a very polite way of declining an invitation. :) So, for example... "Unfortunately I am busy that evening, therefore I must decline your invitation." "I'm afraid that I will have guests at that time, therefore I cannot accept your invitation." "I'm sorry, but I already have other plans." (This is much less formal, but still very kind and polite.)

Also, the response may be different, depending on how formal or informal the correspondence is (e.g. responding to an old friend vs. a client from work).

"I am forced etc. " sound very formal and rather fashioned. You could say "Unfortunately, I cannot accept your invitation" or "I am sorry I cannot...."

???

Thanx but no Thanx